diff --git a/__main__.py b/__main__.py
index 53c2c43..61b23d8 100644
--- a/__main__.py
+++ b/__main__.py
@@ -1,5 +1,18 @@
+import doomsday.date as date
+import doomsday.algorithm as algorithm
+
+
 def main() -> None:
-    print("Hello world")
+    """main function ask a date, if the date is incorrect ask again,
+    when the date is correct,
+    main function ask for the day of the date and return it to the user"""
+
+    while True:
+        input_date: str = input(
+            "Please enter a date if you want to know it day on week")
+        if (date.is_valid_date(input_date)):
+            break
+    print(input_date + " is a " + algorithm.get_weekday_for_date(input_date))
 
 
 main()
diff --git a/doomsday/algorithm.py b/doomsday/algorithm.py
index cf81d60..f5d3bc0 100644
--- a/doomsday/algorithm.py
+++ b/doomsday/algorithm.py
@@ -1,2 +1,96 @@
+from doomsday.date import is_leap, Date_part
+
+WEEKDAY = ("Sunday", "Monday", "Tuesday", "Wednesday",
+           "Thursday", "Friday", "Saturday")
+
+CHANGE_FOR_CENTURY = (2, 0, 5, 3)
+ANCHOR_DAY_IN_MONTH = (10, 21, 0, 4, 9, 6, 11, 8, 5, 10, 7, 12)
+
+WEEK_SIZE = 7
+CENTURY = 100
+
+"""YEAR = 0
+MONTH = 1
+DAY = 2"""
+
+
 def get_weekday_for_date(date: str) -> str:
-    return "Sunday"
+    """get_weekday_for_date need the anchor date
+    and then find the day of the date"""
+
+    # We split again the date
+    list_date: list[str] = date.split('-')
+
+    # We find the anchor day
+    anchor_day_week: int = get_anchor_day(int(list_date[Date_part.YEAR.value]))
+
+    # With the month we find the anchor day in the month
+    anchor_day_in_date_month = find_anchor_day_in_date_month(
+        int(list_date[Date_part.MONTH.value]),
+        int(list_date[Date_part.YEAR.value]))
+
+    # We return the str day in week
+    return get_day_in_week(anchor_day_week,
+                           anchor_day_in_date_month, list_date)
+
+
+def get_anchor_day(year: int) -> int:
+    """get_anchor_day find the anchor day with different operation"""
+
+    # We keep only the two last number of the year
+    number_for_day: int = year % CENTURY
+
+    # We transform this number
+    number_for_day = add_eleven_if_is_odd(number_for_day)
+    number_for_day //= 2
+    number_for_day = add_eleven_if_is_odd(number_for_day)
+    number_for_day = get_difference_near_multiple_of_seven(number_for_day)
+    number_for_day = add_century_step(
+        number_for_day, year // CENTURY * CENTURY) % WEEK_SIZE
+
+    # We find the day in a tab
+    return number_for_day
+
+
+def add_eleven_if_is_odd(number: int) -> int:
+    """We add eleven if number is odd"""
+    return number if number % 2 == 0 else number + 11
+
+
+def get_difference_near_multiple_of_seven(number: int) -> int:
+    """We take difference between number and multiple of seven above"""
+    return get_multiple_of_seven_above(number)-number
+
+
+def get_multiple_of_seven_above(number: int) -> int:
+    """We find the multiple of seven above the number"""
+    number = number // WEEK_SIZE * WEEK_SIZE
+    if number % WEEK_SIZE != 0:
+        number += WEEK_SIZE
+    return number
+
+
+def add_century_step(number: int, century: int) -> int:
+    """we add a number different with the century"""
+    return number + CHANGE_FOR_CENTURY[(century // CENTURY) %
+                                       len(CHANGE_FOR_CENTURY)]
+
+
+def find_anchor_day_in_date_month(month: int, year: int) -> int:
+    """anchor day in month who is in a tab"""
+    if (month < 3 and is_leap(year)):
+        return ANCHOR_DAY_IN_MONTH[month-1] + 1
+    return ANCHOR_DAY_IN_MONTH[month-1]
+
+
+def get_difference_in_day(current_day: int, anchor_day: int) -> int:
+    """get difference between anchor day and current day"""
+    return current_day - anchor_day
+
+
+def get_day_in_week(anchor_day: int, anchor_day_in_date_month: int,
+                    list_date: list[str]) -> str:
+    """get_day_in_week translate anchor to day we search with distance"""
+    distance_in_day = get_difference_in_day(
+        int(list_date[Date_part.DAY.value]), anchor_day_in_date_month)
+    return WEEKDAY[(anchor_day+distance_in_day) % WEEK_SIZE]
diff --git a/doomsday/date.py b/doomsday/date.py
index 0f8e737..10bfa8c 100644
--- a/doomsday/date.py
+++ b/doomsday/date.py
@@ -1,2 +1,114 @@
+from enum import Enum, unique, auto
+
+
+@unique
+class Error(Enum):
+    WRONG_FORMAT = auto()
+    NOT_NUMBER = auto()
+    DATE_NOT_EXIST = auto()
+    YEAR_BEFORE_1583 = auto()
+
+
+@unique
+class Last_day(Enum):
+    SPECIAL_MONTH = 28
+    SPECIAL_MONTH_LEAP = 29
+    MAX_MONTH = 31
+    MIN_MONTH = 30
+
+
+@unique
+class Date_part(Enum):
+    YEAR = 0
+    MONTH = 1
+    DAY = 2
+
+
 def is_valid_date(date: str) -> bool:
+    """is_valid_date test conditions for have a correct date  """
+
+    date_list: list[str] = date.split('-')
+    # Verify the format of the date 'YYYY-MM-DD'
+    if (len(date_list) != 3):
+        return show_error(Error.WRONG_FORMAT, date)
+
+    # Verify that each part of the date is int
+    date_list_int: list[int] = []
+    for element in date_list:
+        if (not element.isnumeric()):
+            return show_error(
+                Error.NOT_NUMBER, date_list[0])
+        date_list_int.append(int(element))
+
+    # Verify that the date is possible Month(1-12) Day(1-lastday)
+    if (date_list_int[Date_part.MONTH.value] < 1 or
+       date_list_int[Date_part.MONTH.value] > 12):
+        return show_error(
+            Error.DATE_NOT_EXIST, date_list[Date_part.MONTH.value])
+
+    if (date_list_int[Date_part.DAY.value] < 1 or
+       date_list_int[Date_part.DAY.value] >
+       find_month_last_day(date_list_int[Date_part.MONTH.value],
+                           date_list_int[Date_part.YEAR.value])):
+        return show_error(
+            Error.DATE_NOT_EXIST, date_list[Date_part.DAY.value])
+
+    # Verify that the year of the date is after 1583
+    if (date_list_int[Date_part.YEAR.value] < 1583):
+        return show_error(
+            Error.YEAR_BEFORE_1583, date_list[Date_part.YEAR.value])
+
     return True
+
+
+def show_error(error_code: Error, error_text: str) -> bool:
+    """create an error message with a code and a text"""
+
+    if (error_code == Error.WRONG_FORMAT):
+        print('Erreur, ' +
+              error_text +
+              ' n est pas un bon format pour une date.' +
+              ' Le format demandé est YYYY-MM-DD.')
+
+    if (error_code == Error.NOT_NUMBER):
+        print('Erreur, ' +
+              error_text +
+              ' n est pas un nombre.')
+
+    if (error_code == Error.DATE_NOT_EXIST):
+        print('Erreur, ' +
+              error_text +
+              ' le mois ou le jour sont incorrects.')
+
+    if (error_code == Error.YEAR_BEFORE_1583):
+        print('Erreur, ' +
+              error_text +
+              ' l année ne doit pas etre en dessous de 1583.')
+
+    return False
+
+
+def find_month_last_day(month_number: int, year: int) -> int:
+    """month_last_day take number of the month
+    and return the last day of month"""
+
+    # Case 31
+    if ((month_number % 2 == 1 and month_number <= 7) or
+       (month_number % 2 == 0 and month_number >= 8)):
+        return Last_day.MAX_MONTH.value
+
+    # Case 28-29
+    if (month_number == 2):
+        if (is_leap(year)):
+            return Last_day.SPECIAL_MONTH_LEAP.value
+        return Last_day.SPECIAL_MONTH.value
+
+    # Case 30
+    return Last_day.MIN_MONTH.value
+
+
+def is_leap(year: int) -> bool:
+    """return if the year is leap"""
+    if year % 4 == 0 and ((not year % 100 == 0) or year % 400 == 0):
+        return True
+    return False