diff --git a/__main__.py b/__main__.py
index 53c2c43..2723d1c 100644
--- a/__main__.py
+++ b/__main__.py
@@ -1,5 +1,19 @@
-def main() -> None:
-    print("Hello world")
+from doomsday.algorithm import get_weekday_for_date
+from doomsday.date import is_valid_date
 
 
-main()
+def main():
+    while True:
+        user_input = input(
+            "Please enter a date using the format YYYY-MM-dd : ")
+
+        # Check date input
+        if is_valid_date(user_input):
+            break
+
+    print(
+        f"{user_input} was a {get_weekday_for_date(user_input)}")
+
+
+if __name__ == "__main__":
+    main()
diff --git a/doomsday/algorithm.py b/doomsday/algorithm.py
index cf81d60..1ba4d3e 100644
--- a/doomsday/algorithm.py
+++ b/doomsday/algorithm.py
@@ -1,2 +1,63 @@
-def get_weekday_for_date(date: str) -> str:
-    return "Sunday"
+from datetime import datetime
+
+WEEKDAYS = (
+    'Sunday',
+    'Monday',
+    'Tuesday',
+    'Wednesday',
+    'Thursday',
+    'Friday',
+    'Saturday'
+)
+# List of memorable dates that always land on doomsday for each month
+# (except January and February)
+doomsday_month: list[int] = [3, 28, 14, 4, 9, 6, 11, 8, 5, 10, 7, 12]
+
+
+def get_weekday_for_date(date_str: str) -> str:
+    """
+    Returns the day of the week for the given date in the format YYYY-MM-dd.
+    """
+    # Convert date to datetime
+    date = datetime.strptime(date_str, '%Y-%m-%d')
+    # Get the doomsday of the year
+    doomsday_of_year = get_doomsday_year(date.year)
+
+    # Check if the year is a leap year
+    # If it is, change the doomsday for January and February
+    if date.year % 400 == 0 or (date.year % 100 != 0 and date.year % 4 == 0):
+        doomsday_month[0] = 4
+        doomsday_month[1] = 29
+
+    # Get the difference between the doomsday of the month and the given date
+    difference_doomsday_and_date = date.day - doomsday_month[date.month - 1]
+
+    # Get the weekday of the date as an integer between 0 and 6
+    weekday = (doomsday_of_year + difference_doomsday_and_date) % 7
+
+    # Return the weekday as a string
+    return WEEKDAYS[weekday]
+
+
+def get_doomsday_year(year: int) -> int:
+    """Gets the doomsday of the year as an integer between 0 and 6"""
+    last_two_digits_of_year: int = year % 100
+
+    if last_two_digits_of_year % 2 != 0:
+        last_two_digits_of_year += 11
+    last_two_digits_of_year //= 2
+    if last_two_digits_of_year % 2 != 0:
+        last_two_digits_of_year += 11
+
+    # Get the difference between the last two numbers of the year and the
+    # closest superior or equal multiple of 7
+    difference_multiple_of_7 = last_two_digits_of_year % 7
+    if difference_multiple_of_7 != 0:
+        difference_multiple_of_7 = 7 - difference_multiple_of_7
+
+    # Number to add to the difference to get the doomsday of the year
+    numbers_to_add_by_century: list[int] = [2, 0, 5, 3]
+
+    # Return the doomsday of the year as an integer between 0 and 6
+    return (difference_multiple_of_7 + numbers_to_add_by_century[
+        (year // 100) % 4]) % 7
diff --git a/doomsday/date.py b/doomsday/date.py
index 0f8e737..e29521b 100644
--- a/doomsday/date.py
+++ b/doomsday/date.py
@@ -1,2 +1,30 @@
-def is_valid_date(date: str) -> bool:
-    return True
+def is_valid_date(date_str: str) -> bool:
+    from datetime import datetime
+
+    try:
+        date_obj: datetime = datetime.strptime(date_str, '%Y-%m-%d')
+
+        # Check if the year is greater than or equal to 1583
+        # If it is not, return False and print an error message
+        if date_obj.year < 1583:
+            print("The year must be greater than or equal to 1583.")
+
+            return False
+
+        return True
+
+    # If the date is not in the correct format (YYYY-MM-dd) or if the date
+    # doesn't exist (for example : 2021-18-45), then return False and print
+    # an error message
+    except ValueError:
+        print(
+            f"Incorrect date {date_str}."
+            " Please use the format YYYY-MM-dd or check if the date exists.")
+
+        return False
+
+    except Exception as e:
+        # Handle any other exception
+        print(e)
+
+        return False