diff --git a/__main__.py b/__main__.py
index 53c2c43..b25a3c5 100644
--- a/__main__.py
+++ b/__main__.py
@@ -1,5 +1,15 @@
+import doomsday.date as date
+import doomsday.algorithm as algorithm
+
+
 def main() -> None:
-    print("Hello world")
+    date_string: str = input("Date (format YYYY-MM-DD): ")
+
+    if (not date.is_valid_date(date_string)):
+        return
+
+    weekday_at_given_date: str = algorithm.get_weekday_for_date(date_string)
+    print(f"The date {date_string} is a {weekday_at_given_date}.")
 
 
 main()
diff --git a/doomsday/algorithm.py b/doomsday/algorithm.py
index cf81d60..d3ad35b 100644
--- a/doomsday/algorithm.py
+++ b/doomsday/algorithm.py
@@ -1,2 +1,65 @@
+import math
+import doomsday.date as date
+
+CENTURY_OFFSETS = (2, 0, 5, 3)
+DAY_NAMES = (
+    "Sunday",
+    "Monday",
+    "Tuesday",
+    "Wednesday",
+    "Thursday",
+    "Friday",
+    "Saturday"
+)
+
+
 def get_weekday_for_date(date: str) -> str:
-    return "Sunday"
+    """Returns the name of the day at a given date."""
+    date_split: list[str] = date.split("-")
+    year: int = int(date_split[0])
+    month: int = int(date_split[1])
+    day: int = int(date_split[2])
+
+    year_anchor_day: int = get_anchor_day_for_year(year)
+    month_anchor: int = get_anchor_for_month(month, year)
+
+    # Calculate the number of days between the month's anchor day and
+    # the wanted day.
+    day_offset: int = abs(day - month_anchor)
+    # Since the month's anchor day and the year's anchor day are the same
+    # we can add the year's anchor day index to get the final day index.
+    final_day_index = (year_anchor_day + day_offset) % 7
+
+    return DAY_NAMES[final_day_index]
+
+
+def get_anchor_day_for_year(year: int) -> int:
+    """Returns the anchor day of a given year starting at 0 for Sunday"""
+
+    # Apply the Odd + 11 algorithm
+    year_digits: int = year % 100
+    if (year_digits % 2 == 1):
+        year_digits += 11
+
+    year_digits = year_digits // 2
+    if (year_digits % 2 == 1):
+        year_digits += 11
+
+    # Calculate the century offset for the current year
+    multiple_of_7: int = math.ceil(year_digits / 7) * 7
+    century_index: int = (year // 100) % 4
+    century_offset = CENTURY_OFFSETS[century_index]
+
+    return multiple_of_7 - year_digits + century_offset
+
+
+def get_anchor_for_month(month: int, year: int) -> int:
+    """Returns the anchor day of a given month given the year"""
+
+    month_anchors = (
+        11 if date.is_leap_year(year) else 10,
+        22 if date.is_leap_year(year) else 21,
+        0, 4, 9, 6, 11, 8, 7, 10, 7, 12
+    )
+
+    return month_anchors[month - 1]
diff --git a/doomsday/date.py b/doomsday/date.py
index 0f8e737..f90d907 100644
--- a/doomsday/date.py
+++ b/doomsday/date.py
@@ -1,2 +1,66 @@
+DAYS_PER_MONTH = (31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31)
+
+
 def is_valid_date(date: str) -> bool:
+    """
+    Returns whether a date is properly formatted to the YYYY-MM-DD format and
+    can be ran through the doomsday algorithm.
+    """
+
+    split_date: list[str] = date.split("-")
+
+    # Check for correct date format
+    if (len(split_date) != 3 or "" in split_date):
+        print("Date must be formatted as YYYY-MM-DD")
+        return False
+
+    year: int = int(split_date[0])
+    month: int = int(split_date[1])
+    day: int = int(split_date[2])
+
+    if (not date_exists(year, month, day)):
+        return False
+
+    # Years below 1583 are not supported.
+    if (year < 1583):
+        print("For simplicity, years before 1583 are not supported.")
+        return False
+
     return True
+
+
+def date_exists(year: int, month: int, day: int) -> bool:
+    """
+    Checks whether the given date can be ran into the doomsday algorithm.
+    Invalid dates are dates before 1583-01-01.
+    """
+
+    # Check for valid month
+    if (month < 1 or month > 12):
+        print("Invalid month")
+        return False
+
+    # Check for valid day, accounting for leap years.
+    if (day < 1 or day > get_days_per_month(month, year)):
+        print("Invalid day number for the given date.")
+        return False
+
+    return True
+
+
+def get_days_per_month(month: int, year: int) -> int:
+    """Returns the number of day of the given month,
+    accounting for leap years"""
+
+    if (month == 2 and is_leap_year(year)):
+        return DAYS_PER_MONTH[month - 1] + 1
+    else:
+        return DAYS_PER_MONTH[month - 1]
+
+
+def is_leap_year(year: int) -> bool:
+    """Check if a year is a leap year.
+    Leap years are every year that are multiples of 4 but not multiples of 100
+    or years that are multiple of 400."""
+
+    return (year % 4 == 0 and year % 100 != 0) or year % 400 == 0