diff --git a/__main__.py b/__main__.py
index 53c2c43..0edf0f9 100644
--- a/__main__.py
+++ b/__main__.py
@@ -1,5 +1,30 @@
+from doomsday.date import is_valid_date
+from doomsday.algorithm import get_weekday_for_date
+
+
 def main() -> None:
-    print("Hello world")
+    """Main function asking for a date and displaying its day"""
+
+    date: str = ask_for_valid_date()
+
+    display_weekday_for_date(date, get_weekday_for_date(date))
+
+
+def ask_for_valid_date() -> str:
+    """Ask for a date until a valid one is given"""
+
+    given_date: str = ""
+
+    while True:
+        given_date = input("Your date (YYYY-MM-dd):\n")
+        if is_valid_date(given_date):
+            return given_date
+
+
+def display_weekday_for_date(date: str, day: str) -> None:
+    """Display a sentence saying a date and its day"""
+
+    print(f"{date} is a {day}")
 
 
 main()
diff --git a/doomsday/algorithm.py b/doomsday/algorithm.py
index cf81d60..3c952f1 100644
--- a/doomsday/algorithm.py
+++ b/doomsday/algorithm.py
@@ -1,2 +1,75 @@
+from doomsday.date import is_leap_year
+
+DAYS = (
+     "Sunday",
+     "Monday",
+     "Tuesday",
+     "Wednesday",
+     "Thursday",
+     "Friday",
+     "Saturday"
+)
+
+CENTURY_ANCHOR = (
+    3, 2, 0, 5
+)
+
+
 def get_weekday_for_date(date: str) -> str:
-    return "Sunday"
+    """Return the name of the day corresponding to the given date"""
+
+    year, month, day = (value for value in date.split("-"))
+    anchor_day: int = get_anchor_day(year)
+
+    return get_weekday_by_anchor_day(
+        int(year), int(month), int(day), anchor_day)
+
+
+def get_anchor_day(year: str) -> int:
+    """Return the anchor day for the given year"""
+
+    # Get the last two numbers in the year
+    anchor_day: int = int(year[-2:len(year)])
+    century: int = int(year[0:-2]) + 1
+
+    if anchor_day % 2 != 0:
+        anchor_day += 11
+
+    anchor_day //= 2
+
+    if anchor_day % 2 != 0:
+        anchor_day += 11
+
+    # Find the lowest multiple of seven greater than the value we have,
+    # and keep the difference between this multiple and our value
+    anchor_day %= 7
+    anchor_day -= 7
+    anchor_day = -anchor_day
+
+    anchor_day += CENTURY_ANCHOR[(century % 4)]
+
+    return anchor_day % 7
+
+
+def get_weekday_by_anchor_day(
+          year: int, month: int, day: int, anchor_day: int) -> str:
+    """Return the weekday of the given MM-dd
+    with the anchor day for the year"""
+
+    is_leap_year_value: bool = is_leap_year(year)
+
+    doomsdays = [
+        3 if not is_leap_year_value else 4,
+        28 if not is_leap_year_value else 29,
+        14, 4, 9, 6, 11, 8, 5, 10, 7, 12
+    ]
+
+    doomsday: int = doomsdays[month - 1]
+
+    if doomsday == day:
+        return DAYS[anchor_day]
+
+    if day > doomsday:
+        return DAYS[(anchor_day + (day - doomsday)) % 7]
+
+    return DAYS[(anchor_day - (doomsday - day)) % 7]
diff --git a/doomsday/date.py b/doomsday/date.py
index 0f8e737..db2a75a 100644
--- a/doomsday/date.py
+++ b/doomsday/date.py
@@ -1,2 +1,61 @@
+DAYS_PER_MONTH = (
+    31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31
+)
+
+
 def is_valid_date(date: str) -> bool:
+    """Check if a date is valid. Which means beign 3 numbers separated by "-",
+    and that the year, month and day given exist in the Gregorian calendar"""
+
+    if not is_valid_format_date(date):
+        return False
+
+    year, month, day = (int(value) for value in date.split("-"))
+
+    if year < 1583:
+        print("Years begin from 1583")
+        return False
+
+    if (1 > month) or (month > 12):
+        print("Month goes from 1 to 12")
+        return False
+
+    days_in_this_month: int = get_days_in_month(year, month)
+
+    if (1 > day) or (day > days_in_this_month):
+        print("For this month, day goes from 1 to ", days_in_this_month)
+        return False
+
     return True
+
+
+def is_valid_format_date(date: str) -> bool:
+    """Verify if a given date is 3 numbers separated by "-" """
+
+    splited_date: list[str] = date.split("-")
+
+    if len(splited_date) != 3:
+        print("Date format should be YYYY-MM-dd")
+        return False
+
+    for part in splited_date:
+        if not part.isdigit():
+            print("A date must be composed with numbers")
+            return False
+
+    return True
+
+
+def get_days_in_month(year: int, month: int) -> int:
+    """Return the number of days in a month, depending on year"""
+
+    if month == 2:
+        return 28 if not is_leap_year(year) else 29
+
+    return DAYS_PER_MONTH[month-1]
+
+
+def is_leap_year(year: int) -> bool:
+    """Verify if a year is a leap year or not"""
+
+    return year % 4 == 0 and (year % 100 != 0 or year % 400 == 0)