diff --git a/src/app/api/dashboard/year/route.test.ts b/src/app/api/dashboard/year/route.test.ts
index ff42b61f..954656b1 100644
--- a/src/app/api/dashboard/year/route.test.ts
+++ b/src/app/api/dashboard/year/route.test.ts
@@ -76,7 +76,7 @@ describe("GET /api/dashboard/year validation", () => {
     it("returns 200 and fetches data when year is valid", async () => {
         vi.mocked(getAuthenticatedUser).mockResolvedValueOnce({ username: "alice", token: "token" });
 
-        vi.mocked(fetchYearInReviewData).mockResolvedValueOnce({ data: "ok" } as unknown as YearInReviewData);
+        vi.mocked(fetchYearInReviewData).mockResolvedValueOnce({ data: "ok", mostActiveDay: null } as unknown as YearInReviewData);
 
         const { GET } = await import("./route");
         const currentYear = new Date().getUTCFullYear();
@@ -85,14 +85,14 @@ describe("GET /api/dashboard/year validation", () => {
 
         expect(response.status).toBe(200);
         const data = await response.json();
-        expect(data).toEqual({ data: "ok" });
+        expect(data).toEqual({ data: "ok", mostActiveDay: null });
         expect(fetchYearInReviewData).toHaveBeenCalledWith("alice", currentYear, "token");
     });
 
     it("returns 200 and falls back to current year when year is not provided", async () => {
         vi.mocked(getAuthenticatedUser).mockResolvedValueOnce({ username: "alice", token: "token" });
 
-        vi.mocked(fetchYearInReviewData).mockResolvedValueOnce({ data: "ok" } as unknown as YearInReviewData);
+        vi.mocked(fetchYearInReviewData).mockResolvedValueOnce({ data: "ok", mostActiveDay: null } as unknown as YearInReviewData);
 
         const { GET } = await import("./route");
         const req = createMockRequest(`http://localhost/api/dashboard/year`);
@@ -100,7 +100,7 @@ describe("GET /api/dashboard/year validation", () => {
 
         expect(response.status).toBe(200);
         const data = await response.json();
-        expect(data).toEqual({ data: "ok" });
+        expect(data).toEqual({ data: "ok", mostActiveDay: null });
 
         const currentYear = new Date().getUTCFullYear();
         expect(fetchYearInReviewData).toHaveBeenCalledWith("alice", currentYear, "token");
diff --git a/src/components/ContributionsCard.tsx b/src/components/ContributionsCard.tsx
index 295cd5b6..860693f0 100644
--- a/src/components/ContributionsCard.tsx
+++ b/src/components/ContributionsCard.tsx
@@ -139,8 +139,6 @@ export default function ContributionsCard({ contributions }: Props) {
   }
 
   const stats = getStats(contributions);
-  const showMostActiveDay = contributions.mostActiveDay.length > 0;
-
   return (
     <div className="glass-card rounded-xl p-6 h-full flex flex-col">
       <h3 className="mb-6 text-lg font-semibold text-foreground flex items-center gap-2">
@@ -153,7 +151,7 @@ export default function ContributionsCard({ contributions }: Props) {
         {stats.map((stat, i) => (
           <StatCard key={stat.label} stat={stat} index={i} />
         ))}
-        {showMostActiveDay && (
+        {contributions.mostActiveDay && (
           <MostActiveDayCard day={contributions.mostActiveDay} />
         )}
       </div>
diff --git a/src/components/YearInReviewCarousel.test.tsx b/src/components/YearInReviewCarousel.test.tsx
index 0535d7d2..d51f69e2 100644
--- a/src/components/YearInReviewCarousel.test.tsx
+++ b/src/components/YearInReviewCarousel.test.tsx
@@ -58,6 +58,15 @@ describe("YearInReviewCarousel", () => {
     expect(screen.getByText("Most active on Monday around 14:00 UTC.")).toBeDefined();
   });
 
+  it("does not stringify null mostActiveDay in the rhythm caption", () => {
+    render(<YearInReviewCarousel data={{ ...mockData, mostActiveDay: null }} />);
+
+    fireEvent.click(screen.getByRole("button", { name: "Prev" }));
+
+    expect(screen.getByText("No most active day yet; your peak activity hour is 14:00 UTC.")).toBeDefined();
+    expect(screen.queryByText(/Most active on null/)).toBeNull();
+  });
+
   it("wraps around to the first slide when 'Next' is clicked on the last slide", () => {
     render(<YearInReviewCarousel data={mockData} />);
 
diff --git a/src/components/YearInReviewCarousel.tsx b/src/components/YearInReviewCarousel.tsx
index 326a1fe3..8c3a56d1 100644
--- a/src/components/YearInReviewCarousel.tsx
+++ b/src/components/YearInReviewCarousel.tsx
@@ -28,7 +28,9 @@ export default function YearInReviewCarousel({ data }: Props) {
       {
         key: "rhythm",
         title: "Your Working Rhythm",
-        caption: `Most active on ${data.mostActiveDay} around ${data.mostActiveHour}:00 UTC.`,
+        caption: data.mostActiveDay
+          ? `Most active on ${data.mostActiveDay} around ${data.mostActiveHour}:00 UTC.`
+          : `No most active day yet; your peak activity hour is ${data.mostActiveHour}:00 UTC.`,
       },
     ],
     [data],
diff --git a/src/components/YearInReviewSlide.test.tsx b/src/components/YearInReviewSlide.test.tsx
index c7e96ece..ef86b012 100644
--- a/src/components/YearInReviewSlide.test.tsx
+++ b/src/components/YearInReviewSlide.test.tsx
@@ -62,4 +62,10 @@ describe("YearInReviewSlide", () => {
     expect(screen.getByText("Most active day: Monday")).toBeInTheDocument();
     expect(screen.queryByText(/Top repo:/)).not.toBeInTheDocument();
   });
+
+  it("does not render the most active day badge when mostActiveDay is null", () => {
+    render(<YearInReviewSlide title="Title" data={{ ...mockData, mostActiveDay: null }} />);
+
+    expect(screen.queryByText(/Most active day:/)).not.toBeInTheDocument();
+  });
 });
diff --git a/src/components/YearInReviewSlide.tsx b/src/components/YearInReviewSlide.tsx
index ae2d3c36..88f296cb 100644
--- a/src/components/YearInReviewSlide.tsx
+++ b/src/components/YearInReviewSlide.tsx
@@ -57,9 +57,11 @@ export default function YearInReviewSlide({ title, caption, data }: Props) {
       </div>
 
       <div className="mt-6 flex flex-wrap items-center gap-3 text-sm">
-        <span className="rounded-full bg-success/15 px-3 py-1 text-success">
-          Most active day: {data.mostActiveDay}
-        </span>
+        {data.mostActiveDay ? (
+          <span className="rounded-full bg-success/15 px-3 py-1 text-success">
+            Most active day: {data.mostActiveDay}
+          </span>
+        ) : null}
         {data.topRepository ? (
           <span className="rounded-full bg-accent/15 px-3 py-1 text-accent">
             Top repo: {data.topRepository.name} (
diff --git a/src/components/__tests__/ContributionsCard.test.tsx b/src/components/__tests__/ContributionsCard.test.tsx
index 7c74729f..7537127f 100644
--- a/src/components/__tests__/ContributionsCard.test.tsx
+++ b/src/components/__tests__/ContributionsCard.test.tsx
@@ -16,7 +16,7 @@ describe("ContributionsCard", () => {
           totalReviews: 0,
           longestStreak: 0,
           currentStreak: 0,
-          mostActiveDay: "",
+          mostActiveDay: null,
           calendar: [],
         }}
       />
@@ -44,4 +44,26 @@ describe("ContributionsCard", () => {
 
     expect(screen.getByText("Contributions")).toBeInTheDocument();
   });
+
+  it("does not render MostActiveDayCard when mostActiveDay is null", () => {
+    render(
+      <ContributionsCard
+        contributions={{
+          totalContributions: 10, monthlyContributions: 0, weeklyContributions: 0,
+          totalCommits: 5,
+          totalPRs: 3,
+          totalIssues: 1,
+          totalReviews: 1,
+          longestStreak: 5,
+          currentStreak: 2,
+          mostActiveDay: null,
+          calendar: [],
+        }}
+      />
+    );
+
+    expect(screen.getByText("Contributions")).toBeInTheDocument();
+    expect(screen.queryByText("Most Active Day")).not.toBeInTheDocument();
+  });
+
 });
diff --git a/src/lib/__tests__/github/fetchContributions.test.ts b/src/lib/__tests__/github/fetchContributions.test.ts
index 3cff75c0..31e5cc3a 100644
--- a/src/lib/__tests__/github/fetchContributions.test.ts
+++ b/src/lib/__tests__/github/fetchContributions.test.ts
@@ -104,4 +104,42 @@ describe("fetchContributions", () => {
     // monthly is 7 days times 10 plus 23 days times 5 equals 185
     expect(result.monthlyContributions).toBe(185);
   });
+
+  it("空の履歴(contributionDaysが空または全て0)を正しく処理する", async () => {
+    const mockEmptyContributions = {
+      data: {
+        user: {
+          contributionsCollection: {
+            totalCommitContributions: 0,
+            totalPullRequestContributions: 0,
+            totalIssueContributions: 0,
+            totalPullRequestReviewContributions: 0,
+            contributionCalendar: {
+              totalContributions: 0,
+              weeks: [
+                {
+                  contributionDays: []
+                }
+              ],
+            },
+          },
+        },
+      },
+    };
+
+    mockFetch.mockResolvedValueOnce(jsonResponse(mockEmptyContributions));
+
+    const { fetchContributions } = await import("../../github");
+    const result = await fetchContributions("testuser", "fake-token");
+
+    expect(result.totalCommits).toBe(0);
+    expect(result.totalContributions).toBe(0);
+    expect(result.weeklyContributions).toBe(0);
+    expect(result.monthlyContributions).toBe(0);
+    expect(result.longestStreak).toBe(0);
+    expect(result.currentStreak).toBe(0);
+    expect(result.mostActiveDay).toBeNull();
+    expect(result.calendar.length).toBe(0);
+  });
+
 });
diff --git a/src/lib/__tests__/yearInReviewUtils.test.ts b/src/lib/__tests__/yearInReviewUtils.test.ts
index c964d7bc..2dc09118 100644
--- a/src/lib/__tests__/yearInReviewUtils.test.ts
+++ b/src/lib/__tests__/yearInReviewUtils.test.ts
@@ -133,8 +133,8 @@ describe("getMostActiveHour", () => {
 });
 
 describe("getMostActiveDayFromCalendar", () => {
-    it("returns 'Sunday' when the calendar is empty", () => {
-        expect(getMostActiveDayFromCalendar([])).toBe("Sunday");
+    it("returns null when the calendar is empty", () => {
+        expect(getMostActiveDayFromCalendar([])).toBeNull();
     });
 
     it("correctly identifies the most active day of the week", () => {
diff --git a/src/lib/github.ts b/src/lib/github.ts
index 93489f7e..ed7dd37f 100644
--- a/src/lib/github.ts
+++ b/src/lib/github.ts
@@ -88,7 +88,7 @@ function calculateStreaks(calendar: { count: number }[]): { longestStreak: numbe
   return { longestStreak, currentStreak };
 }
 
-function calculateMostActiveDay(calendar: { date: string; count: number }[]): string {
+function calculateMostActiveDay(calendar: { date: string; count: number }[]): string | null {
   const weekdayNames = ["Sunday", "Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday"];
   const weekdayTotals = Array.from({ length: 7 }, () => 0);
 
@@ -103,7 +103,7 @@ function calculateMostActiveDay(calendar: { date: string; count: number }[]): st
   const maxWeekdayTotal = Math.max(...weekdayTotals);
   return maxWeekdayTotal > 0
     ? weekdayNames[weekdayTotals.findIndex((count) => count === maxWeekdayTotal)]
-    : "";
+    : null;
 }
 
 async function graphql<T>(query: string, token?: string, variables?: Record<string, unknown>): Promise<T> {
diff --git a/src/lib/types.ts b/src/lib/types.ts
index e55b3d26..6c6e2af5 100644
--- a/src/lib/types.ts
+++ b/src/lib/types.ts
@@ -59,7 +59,7 @@ export type ContributionData = {
   weeklyContributions: number;
   longestStreak: number;
   currentStreak: number;
-  mostActiveDay: string;
+  mostActiveDay: string | null;
   calendar: { date: string; count: number }[];
 };
 
@@ -91,7 +91,7 @@ export type YearInReviewData = {
   totalPRs: number;
   totalIssues: number;
   totalReviews: number;
-  mostActiveDay: string;
+  mostActiveDay: string | null;
   mostActiveHour: number;
   topRepository: {
     name: string;
diff --git a/src/lib/yearInReviewUtils.ts b/src/lib/yearInReviewUtils.ts
index 52cee3ea..a128daf9 100644
--- a/src/lib/yearInReviewUtils.ts
+++ b/src/lib/yearInReviewUtils.ts
@@ -96,9 +96,9 @@ export function getMostActiveHour(heatmap: number[][]): number {
  * Returns the most active day of the week from the contribution calendar data.
  *
  * @param calendar Array of objects containing date and contribution count.
- * @returns The name of the most active day (e.g., "Monday").
+ * @returns The name of the most active day (e.g., "Monday"), or null if there are no contributions.
  */
-export function getMostActiveDayFromCalendar(calendar: { date: string; count: number }[]): string {
+export function getMostActiveDayFromCalendar(calendar: { date: string; count: number }[]): string | null {
     const weekdayNames = ["Sunday", "Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday"];
     const totals = Array.from({ length: 7 }, () => 0);
 
@@ -121,5 +121,5 @@ export function getMostActiveDayFromCalendar(calendar: { date: string; count: nu
         }
     }
 
-    return weekdayNames[maxDay];
+    return totals[maxDay] > 0 ? weekdayNames[maxDay] : null;
 }