diff --git a/__main__.py b/__main__.py
index 53c2c43..7b5d703 100644
--- a/__main__.py
+++ b/__main__.py
@@ -1,5 +1,15 @@
+from doomsday.date import *
+from doomsday.algorithm import *
+
+
 def main() -> None:
-    print("Hello world")
+
+    while True:
+        date = input("Veuillez saisir une date au format YYYY-MM-DD: ")
+
+        if is_valid_date(date):
+            print(f"Le {date} est un {get_weekday_for_date(date)}.")
+            break
 
 
 main()
diff --git a/doomsday/algorithm.py b/doomsday/algorithm.py
index cf81d60..6a2d29e 100644
--- a/doomsday/algorithm.py
+++ b/doomsday/algorithm.py
@@ -1,2 +1,39 @@
+from doomsday.algorithm_helper_function import *
+from doomsday.does_date_exist import split_date, is_leap_year
+
+
+MONTHS_ANCHOR: list[int] = [10, 21, 0, 4, 9, 6, 11, 8, 5, 10, 7, 12]
+WEEKDAYS: list[str] = ["Sunday", "Monday", "Tuesday", "Wednesday", "Thursday",
+                       "Friday", "Saturday"]
+
+
 def get_weekday_for_date(date: str) -> str:
-    return "Sunday"
+    """Return the weekday from a date
+
+    date has to be of valid format and exist"""
+
+    splitted_date: list[int] = split_date(date)
+
+    year:   int = splitted_date[0]
+    month:  int = splitted_date[1]
+    day:    int = splitted_date[2]
+
+    # Finds the doomsday of a given year
+    doomsday: int = get_year_doomsday(year)
+
+    # Retreive the anchor day of a given month
+    anchor_day: int = MONTHS_ANCHOR[month - 1]
+
+    # During a leap year the anchor for January and February changes
+    # January: 10 -> 11      February: 21 -> 22
+    if is_leap_year(year) and (month == 1 or month == 2):
+        anchor_day += 1
+
+    # Get the difference between the anchor day and the choosen day
+    diff_with_anchor: int = day - anchor_day
+
+    # Adds the doomsday to our diff as an offset and apply mod 7
+    # to get our final answer
+    weekday: int = (diff_with_anchor + doomsday) % 7
+
+    return WEEKDAYS[weekday]
diff --git a/doomsday/algorithm_helper_function.py b/doomsday/algorithm_helper_function.py
new file mode 100644
index 0000000..abbc0e0
--- /dev/null
+++ b/doomsday/algorithm_helper_function.py
@@ -0,0 +1,40 @@
+def get_century_doomsday_offset(year: int) -> int:
+    """Returns the doomsday offset for a given century"""
+
+    # List of the offsets and exemples
+    # 2000      2
+    # 1700/2100 0
+    # 1800/2200 5
+    # 1900/2300 3
+    CENTURY_OFFSETS: list[int] = [2, 0, 5, 3]
+
+    century: int = year // 100
+
+    return CENTURY_OFFSETS[century % 4]
+
+
+def get_year_doomsday(year: int) -> int:
+    """Return the doomsday for a given year"""
+
+    century_offset: int = get_century_doomsday_offset(year)
+
+    # The algorithm to find the doomsday for a year:
+    # 1: Get the last two digits of the year
+    last_2_digit: int = year % 100
+
+    # 2: If the number is odd add 11
+    if last_2_digit % 2 == 1:
+        last_2_digit += 11
+
+    # 3: Divide the number by 2
+    last_2_digit = last_2_digit // 2
+
+    # 4: If the number is odd add 11
+    if last_2_digit % 2 == 1:
+        last_2_digit += 11
+
+    # 5: Find the difference with the next multiple of 7
+    diff_with_next_multiple_of_seven: int = (7 - (last_2_digit % 7)) % 7
+
+    # 6: Add the century offset and apply mod 7, you have the doomsday
+    return (diff_with_next_multiple_of_seven + century_offset) % 7
diff --git a/doomsday/date.py b/doomsday/date.py
index 0f8e737..72316af 100644
--- a/doomsday/date.py
+++ b/doomsday/date.py
@@ -1,2 +1,27 @@
+import re
+from doomsday.does_date_exist import *
+
+
 def is_valid_date(date: str) -> bool:
+    """Verify if the date parameter is valid
+
+    First the function verify the format,
+    then it verify if the date exist
+    """
+
+    if not is_format_valid(date):
+        print("Le format de la date n'est pas YYYY-MM-DD!")
+        return False
+
+    if not does_date_exist(date):
+        print("La date n'existe pas!")
+        return False
+
     return True
+
+
+def is_format_valid(date: str) -> bool:
+    """Verify if date follows the format YYYY-MM-DD"""
+    format_match = re.fullmatch(r"\d{4}[-]\d{1,2}[-]\d{1,2}", date)
+
+    return format_match is not None
diff --git a/doomsday/does_date_exist.py b/doomsday/does_date_exist.py
new file mode 100644
index 0000000..8727013
--- /dev/null
+++ b/doomsday/does_date_exist.py
@@ -0,0 +1,58 @@
+LAST_DAY_OF_MONTH: list[int] = [31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31]
+
+
+def does_date_exist(date: str) -> bool:
+    """Verify if the date exist
+
+    date has to be of valid format (YYYY-MM-DD)"""
+
+    splitted_date: list[int] = split_date(date)
+
+    year:   int = splitted_date[0]
+    month:  int = splitted_date[1]
+    day:    int = splitted_date[2]
+
+    # We support only dates starting from 1583
+    if year < 1583:
+        print("L'année saisit n'est pas supportée.")
+        print("Veuillez saisir au date à partir de 1583-01-01")
+        return False
+
+    if not (1 <= month <= 12):
+        print("Le mois saisit n'est pas valide.")
+        return False
+
+    if day <= 0:
+        print("Le jour saisit n'est pas valide.")
+        return False
+
+    if day > LAST_DAY_OF_MONTH[month-1]:
+        # If we are in february during a leap year we have 29 days
+        if month == 2 and is_leap_year(year) and day <= 29:
+            return True
+
+        print("Le jour saisit n'est pas valide.")
+        return False
+
+    return True
+
+
+def is_leap_year(year: int) -> bool:
+    """Returns true if year is bisextile"""
+    divisable_by_4 = year % 4 == 0
+    divisable_by_100 = year % 100 == 0
+    divisable_by_400 = year % 400 == 0
+    return divisable_by_4 and (not divisable_by_100 or divisable_by_400)
+
+
+def split_date(date: str) -> list[int]:
+    """Splits a date into a list of ints
+
+    date has to be of valid format (YYYY-MM-DD)"""
+    # Split the date in three parts using the '-' character
+    splitted_date_str: list[str] = date.split("-")
+
+    # Convert the splitted date to ints
+    splitted_date_int: list[int] = list(map(int, splitted_date_str))
+
+    return splitted_date_int