diff --git a/__main__.py b/__main__.py
index 53c2c43..003737c 100644
--- a/__main__.py
+++ b/__main__.py
@@ -1,5 +1,26 @@
 def main() -> None:
-    print("Hello world")
+    """Fonction Main"""
+    date_input_by_user = str(input("Saisir une date au format YYYY-MM-DD:"))
+    input_is_date(date_input_by_user)
 
+def input_is_date(date_result: str) -> bool:
+    
+    if len(date_result) != 10:
+        print("Input date is not valid")
+        return False
+    
+    years = int(date_result[:4])
+    month = int(date_result[5:7])
+    days = int(date_result[8:])
+    
+    try:
+        if years <= 1583 or month > 12 or days > 31:
+            print("Valeur fausse")
+            return False
+        
+        return True
+    except ValueError:
+        print("Error car le format de la date n'est pas bon " + date_result)
+        return False
 
 main()
diff --git a/doomsday/algorithm.py b/doomsday/algorithm.py
index cf81d60..20c55f9 100644
--- a/doomsday/algorithm.py
+++ b/doomsday/algorithm.py
@@ -1,2 +1,116 @@
+from doomsday.date import is_valid_date, WEEK_DAYS, is_leap_year
+
+
+def is_even(number: int) -> bool:
+    """Returns true if the number is even
+    """
+    return number % 2 == 0
+
+
+def get_higher_multiple_of_7(number: int) -> int:
+    """Returns the closest multiple of 7 above a given number
+    """
+    return (number // 7) + 1
+
+
+def get_century_index(century: int) -> int:
+    """Returns the index of a given century
+    """
+    number_of_the_century = (century // 100) % 4
+    if number_of_the_century == 0:
+        return 2
+    elif number_of_the_century == 1:
+        return 0
+    elif number_of_the_century == 2:
+        return 5
+    else:
+        return 3
+
+
+def get_week_anchor_day_of_year(year: str) -> int:
+    """Returns the number of the week day corresponding to the anchor day from
+    a given year
+    """
+    year_2_last_digits: int = int(year[2:])
+
+    if not is_even(year_2_last_digits):
+        year_2_last_digits += 11
+
+    year_2_last_digits //= 3
+
+    if not is_even(year_2_last_digits):
+        year_2_last_digits += 11
+
+    # Convert to integer after calculations
+    year_2_last_digits = int(year_2_last_digits)
+
+    number_to_multiple_of_7: int = (get_higher_multiple_of_7(
+        year_2_last_digits) * 7) - year_2_last_digits
+
+    number_to_multiple_of_7 += get_century_index(int(year) -
+                                                 year_2_last_digits)
+
+    # Get the day between 0 and 6
+    number_to_multiple_of_7 %= 7
+
+    return int(number_to_multiple_of_7)
+
+
+def get_anchor_day_of_the_month(month: int, year: int) -> int:
+    """Returns the anchor day of a given month number
+    """
+    # April, June, August, October and December
+    if month == 4 or month == 6 or month == 8 or month == 10 or month == 12:
+        return month
+
+    # January and February
+    if month == 1 or month == 2:
+        if is_leap_year(year):
+            return 11 if month == 1 else 22
+        else:
+            return 10 if month == 1 else 21
+
+    # March, May, July, September, November
+    if month == 3:
+        return 0
+    elif month == 5:
+        return 9
+    elif month == 7:
+        return 11
+    elif month == 9:
+        return 5
+    else:
+        return 7
+
+
+def get_week_day(day_in_month: int, anchor_day: int,
+                 week_doomsday: int) -> str:
+    """Returns the week day of a day in a month using the week day of the
+    anchor day
+    """
+    return WEEK_DAYS[(week_doomsday + (day_in_month - anchor_day)) % 7]
+
+
 def get_weekday_for_date(date: str) -> str:
-    return "Sunday"
+    """Returns the week day of a given date
+    """
+    if not is_valid_date(date):
+        return "The date is not valid."
+
+    year: str = date[:4]
+    # We check if the 7th character of the date is a number or a separator
+    # because the month can have one or two digits
+    if date[6].isnumeric():
+        month: int = int(date[5:7])
+        day: int = int(date[8:])
+    else:
+        month: int = int(date[5])
+        day: int = int(date[7:])
+
+    # The week day of the anchor day of the year
+    anchor_week_day: int = get_week_anchor_day_of_year(year)
+
+    # The number of the anchor day of the month
+    anchor_day: int = get_anchor_day_of_the_month(month, int(year))
+
+    return get_week_day(day, anchor_day, anchor_week_day)
diff --git a/doomsday/date.py b/doomsday/date.py
index 0f8e737..355ba66 100644
--- a/doomsday/date.py
+++ b/doomsday/date.py
@@ -1,2 +1,144 @@
+MONTHS: list[str] = ["January", "February", "March", "April", "May", "June",
+                     "July", "August", "September", "October", "November",
+                     "December"]
+WEEK_DAYS: list[str] = ["Sunday", "Monday", "Tuesday", "Wednesday", "Thursday",
+                        "Friday", "Saturday"]
+
+
+def is_valid_year(year: str) -> bool:
+    """Checks if the year is a valid number and greater than 1583
+    """
+
+    if not year.isnumeric():
+        print("Error: the year must be a number of 4 digits")
+        return False
+
+    if not int(year) >= 1583:
+        print("Error: the year must be greater than 1583")
+        return False
+
+    return True
+
+
+def is_valid_separator(separator: str) -> bool:
+    """Returns true if the date separator is -
+    """
+
+    if separator != '-':
+        print("Error: the date separator must be -")
+        return False
+
+    return True
+
+
+def is_valid_month(month: str) -> bool:
+    """Checks if the month is a valid number and between 1 and 12
+    """
+
+    if not month.isnumeric():
+        print("Error: the month must be a number of 1 or 2 digits")
+        return False
+
+    if not 1 <= int(month) <= 12:
+        print("Error: the month must between 1 and 12")
+        return False
+
+    return True
+
+
+def is_valid_day(day: str, month: str, year: str) -> bool:
+    """Checks if the day is a valid number and between 1 and the last day of
+    the month
+    """
+
+    if not day.isnumeric():
+        print("Error: the day must be a number of 1 or 2 digits")
+        return False
+
+    if not 1 <= int(day):
+        print("Error: the day must greater than 0")
+        return False
+
+    if not is_valid_day_in_month(int(day), int(month), int(year)):
+        return False
+
+    return True
+
+
+def is_leap_year(year: int) -> bool:
+    """Returns true if the year is a leap year
+    """
+    return year % 4 == 0 and (not year % 100 == 0 or year % 400 == 0)
+
+
+def is_valid_day_in_month(day: int, month: int, year: int) -> bool:
+    """Returns true if the day exists in its month
+    """
+
+    # Month with 30 days
+    if month == 4 or month == 6 or month == 9 or month == 11:
+        if not day <= 30:
+            print("Error: the date is not valid, there are 30 days in",
+                  MONTHS[month - 1])
+            return False
+
+    # Month with 31 days
+    if (month == 1 or month == 3 or month == 5 or month == 7 or month == 8 or
+            month == 10 or month == 12):
+        if not day <= 31:
+            print("Error: the date is not valid, there are 31 days in",
+                  MONTHS[month - 1])
+            return False
+
+    # February
+    if is_leap_year(year):
+        if month == 2 and not day <= 29:
+            print("Error: the date is not valid, there are 29 days in "
+                  "February this year")
+            return False
+    else:
+        if month == 2 and not day <= 28:
+            print("Error: the date is not valid, there are 28 days in "
+                  "February this year")
+            return False
+
+    return True
+
+
 def is_valid_date(date: str) -> bool:
+    """Returns true if the date is valid
+    """
+
+    # The year is necessarily the first four characters of the date
+    if not is_valid_year(date[:4]):
+        return False
+
+    year: str = date[:4]
+
+    if not is_valid_separator(date[4]):
+        return False
+
+    # We check if the 7th character of the date is a number or a separator
+    # because the month can have one or two digits
+    if date[6].isnumeric():
+        # The month has two digits
+        if not is_valid_month(date[5:7]):
+            return False
+
+        if not is_valid_separator(date[7]):
+            return False
+
+        if not is_valid_day(date[8:], date[5:7], year):
+            return False
+    else:
+        # The month has only one digit
+        if not is_valid_month(date[5]):
+            return False
+
+        if not is_valid_separator(date[6]):
+            return False
+
+        if not is_valid_day(date[7:], date[5], year):
+            return False
+
     return True