diff --git a/__main__.py b/__main__.py
index 53c2c43..c68067c 100644
--- a/__main__.py
+++ b/__main__.py
@@ -1,5 +1,23 @@
+from doomsday.algorithm import get_weekday_for_date
+from tests.test_algorithm import TestAlgorithm
+from tests.test_date import TestDate
+
+
 def main() -> None:
-    print("Hello world")
+    dates = ["2000-01-01",
+             "2001-01-29",
+             "2017-12-15",
+             "2023-11-15",
+             "2111-12-12"]  # Saturday, Monday, Friday, Wednesday, Saturday
+    for date in dates:
+        print("{0} is a {1}.".format(date, get_weekday_for_date(date)))
+
+    test_date = TestDate()
+    test_date.test_valid_dates()
+    test_date.test_invalid_dates()
+
+    test_algorithm = TestAlgorithm()
+    test_algorithm.test_algorithm()
 
 
 main()
diff --git a/doomsday/algorithm.py b/doomsday/algorithm.py
index cf81d60..d64b3a8 100644
--- a/doomsday/algorithm.py
+++ b/doomsday/algorithm.py
@@ -1,2 +1,49 @@
+from doomsday.date import is_valid_date
+
+
 def get_weekday_for_date(date: str) -> str:
-    return "Sunday"
+    if not is_valid_date(date):
+        return "Invalid date"
+
+    year, month, day = map(int, date.split('-'))
+    day_of_week = calculate_doomsday(year, month, day)
+
+    week_days = ["Sunday", "Monday",
+                 "Tuesday", "Wednesday",
+                 "Thursday", "Friday", "Saturday"]
+    return week_days[day_of_week]
+
+
+def get_year_anchor(year: int) -> int:
+    # Define the anchor day for the century
+    anchor_day = 2
+
+    # Calculate the century anchor day
+    century = year // 100
+    century_anchor = (5 * (century % 4) + anchor_day) % 7
+
+    # Calculate the year anchor day
+    year_within_century = year % 100
+    year_anchor = (century_anchor + (year_within_century // 12)
+                   + (year_within_century % 12)
+                   + ((year_within_century % 12) // 4)
+                   ) % 7
+
+    return year_anchor
+
+
+def calculate_doomsday(year: int, month: int, day: int) -> int:
+    year_anchor = get_year_anchor(year)
+
+    # Doomsday for each month
+    doomsday_month = [3, 0, 0, 4, 9, 6, 11, 8, 5, 10, 7, 12]
+
+    # Adjustments for January and February in leap years
+    if (year % 4 == 0 and year % 100 != 0) or (year % 400 == 0):
+        doomsday_month[0] = 4
+        doomsday_month[1] = 1
+
+    # Calculate the day of the week for the given date
+    day_of_week = (day - doomsday_month[month - 1] + year_anchor) % 7
+
+    return day_of_week
diff --git a/doomsday/date.py b/doomsday/date.py
index 0f8e737..fc5f9ea 100644
--- a/doomsday/date.py
+++ b/doomsday/date.py
@@ -1,2 +1,22 @@
 def is_valid_date(date: str) -> bool:
-    return True
+    try:
+        year, month, day = map(int, date.split('-'))
+
+        if year < 1583:
+            return False
+
+        if 1 <= month <= 12 and 1 <= day <= 31:
+            if month in [4, 6, 9, 11] and day > 30:
+                return False
+            elif month == 2:  # February
+                if day > 29:
+                    return False
+                elif day == 29 and not (year % 4 == 0 and
+                                        (year % 100 != 0 or year % 400 == 0)):
+                    return False  # February 29 is only valid in leap years
+
+            return True
+    except ValueError:
+        pass
+
+    return False