diff --git a/__main__.py b/__main__.py
index 53c2c43..a23f072 100644
--- a/__main__.py
+++ b/__main__.py
@@ -1,5 +1,14 @@
+from doomsday.algorithm import get_weekday_for_date
+from doomsday.date import is_valid_date
+
+
 def main() -> None:
-    print("Hello world")
+    user_input = input("Please enter a date in the format YYYY-MM-dd: ")
+
+    if is_valid_date(user_input):
+        print(f"The weekday for {user_input} is: {get_weekday_for_date(user_input)}")
+    else:
+        print("Error: The date is not valid.")
 
 
 main()
diff --git a/doomsday/algorithm.py b/doomsday/algorithm.py
index cf81d60..ee46e88 100644
--- a/doomsday/algorithm.py
+++ b/doomsday/algorithm.py
@@ -1,2 +1,52 @@
+DAYS: tuple[str, str, str, str, str, str, str] = (
+    'Sunday',
+    'Monday',
+    'Tuesday',
+    'Wednesday',
+    'Thursday',
+    'Friday',
+    'Saturday'
+)
+
+
 def get_weekday_for_date(date: str) -> str:
-    return "Sunday"
+    """Returns the weekday for the given date in the format YYYY-MM-dd."""
+    year, month, day = (int(i) for i in date.split('-'))
+
+    anchor_day_index: int = get_anchor_of_the_year(year)
+
+    doomsday_month: list[int] = [3, 7, 7, 4, 2, 6, 4, 1, 5, 3, 7, 5]
+
+    if is_leap_year(year):
+        doomsday_month[0] = 4
+        doomsday_month[1] = 1
+
+    # Calculate the difference between the day and the doomsday of the month
+    difference: int = day - doomsday_month[month - 1]
+    # Calculate the weekday index by using the anchor day index and the difference
+    weekday_index: int = (anchor_day_index + difference) % 7
+
+    return DAYS[weekday_index]
+
+
+def is_leap_year(year: int) -> bool:
+    """Checks if the given year is a leap year."""
+    return year % 400 == 0 or (year % 100 != 0 and year % 4 == 0)
+
+
+def get_anchor_of_the_year(year: int) -> int:
+    """Calculates the anchor day index for the given year."""
+    value: int = year % 100
+
+    if value % 2 == 1:
+        value += 11
+    value //= 2
+    if value % 2 == 1:
+        value += 11
+
+    # multiple of 7 equals or greater than century year
+    difference_multiple_of_7: int = (7 - value) % 7
+
+    numbers_to_add_by_century_year: list[int] = [2, 0, 5, 3]
+
+    return (difference_multiple_of_7 + numbers_to_add_by_century_year[(year // 100) % 4]) % 7
diff --git a/doomsday/date.py b/doomsday/date.py
index 0f8e737..3c97f78 100644
--- a/doomsday/date.py
+++ b/doomsday/date.py
@@ -1,2 +1,25 @@
-def is_valid_date(date: str) -> bool:
-    return True
+from datetime import datetime
+
+
+def is_valid_date(date_str: str) -> bool:
+    """Checks if the given date is valid."""
+    try:
+        # Try to convert the string to a date object
+        date_obj: datetime = datetime.strptime(date_str, '%Y-%m-%d')
+
+        # Check if the year is greater than or equal to 1583
+        if date_obj.year < 1583:
+            print("Error: The year must be greater than or equal to 1583.")
+            return False
+
+        return True
+
+    except ValueError:
+        # Conversion error, the date is not in the expected format
+        print("Error: The date does not follow the format YYYY-MM-dd.")
+        return False
+
+    except Exception as e:
+        # Handle other potential errors
+        print(f"An error occurred: {e}")
+        return False