diff --git a/__main__.py b/__main__.py
index 53c2c43..eb598cf 100644
--- a/__main__.py
+++ b/__main__.py
@@ -1,5 +1,21 @@
+from doomsday.date import is_valid_date
+from doomsday.algorithm import get_weekday_for_date
+
 def main() -> None:
-    print("Hello world")
+    
+    stop = False
+
+    while not stop :
+        input_date = input("\n Enter a date and I'll get the day (quit to stop the programm) : ")
+        print("\n")
+
+        if input_date.lower() == "quit" :
+            stop = True
+        else :
+            if is_valid_date(input_date):
+                print("You asked for " + get_weekday_for_date(input_date))
+    
+    print("bye bye")
 
 
 main()
diff --git a/doomsday/algorithm.py b/doomsday/algorithm.py
index cf81d60..424bc82 100644
--- a/doomsday/algorithm.py
+++ b/doomsday/algorithm.py
@@ -1,2 +1,95 @@
+from doomsday.toolbox import is_leap_year
+from doomsday.toolbox import parse_date_to_ints
+
+from doomsday.const import DAYS
+
 def get_weekday_for_date(date: str) -> str:
-    return "Sunday"
+    '''
+        Returns the right day asked from the date (it's magic)
+    '''
+    split_date: list[int] = parse_date_to_ints(date)
+
+    year: int = split_date[0]
+    month: int = split_date[1]
+    day: int = split_date[2]
+
+    doomsday: int = get_doomsday(month, year)
+    anchor_day: int = get_anchor_day(year)
+
+    out_day = int((anchor_day + ((day - doomsday) % 7)) % 7)
+
+    return DAYS()[out_day]
+
+
+def get_anchor_day(year: int) -> int:
+    '''
+        Get the anchor day of the year
+    '''
+
+    anchor = year - ((year // 100) * 100)
+
+    if anchor % 2 != 0 :
+        anchor += 11
+        anchor /= 2
+    
+
+    if anchor % 2 != 0 :
+        anchor += 11
+
+    anchor = 7 - (anchor % 7)
+    century_anchor: int = get_century_anchor(year)
+
+    return anchor + century_anchor
+
+
+def get_century_anchor(year: int) -> int:
+    '''
+        Subshit from get_anchor_day() getting the century anchor
+    '''
+
+    century_basis = year // 100
+
+    if century_basis % 4 == 15 % 4 :
+        return 3
+    elif century_basis % 4 == 16 % 4 :
+        return 2
+    elif century_basis % 4 == 17 % 4 :
+        return 0
+    else :
+        return 5
+
+
+def get_doomsday(month: int, year: int) -> int:
+    '''
+        Get the doomsday of the date
+    '''
+    # april, june, august, october, december
+    if month in [4, 6, 8, 10, 12] :
+        return month
+    
+    # march
+    if month == 3 :
+        return 0
+    
+    # january, february
+    if (month == 1) :
+        if(is_leap_year(year)) :
+            return 11
+        else :
+            return 10
+    
+    #february
+    if (month == 2) :
+        if(is_leap_year(year)) :
+            return 22
+        else :
+            return 21
+
+    # may, july, september, november
+    for tuple in [[5, 9], [7, 11]] :
+        if month in tuple :
+            if tuple.index(month) == 1 :
+                return tuple[0]
+            else :
+                return tuple[1]
+
diff --git a/doomsday/const.py b/doomsday/const.py
new file mode 100644
index 0000000..d8fad97
--- /dev/null
+++ b/doomsday/const.py
@@ -0,0 +1,6 @@
+def PHALANX_MONTH() -> list[int] :
+    return [1, 3, 5, 7, 8, 10, 12]
+
+def DAYS() -> list[str] :
+    return ["Sunday", "Monday", "Tuesday", "Wednesday", 
+            "Thursday", "Friday", "Saturday"]
\ No newline at end of file
diff --git a/doomsday/date.py b/doomsday/date.py
index 0f8e737..9596ecc 100644
--- a/doomsday/date.py
+++ b/doomsday/date.py
@@ -1,2 +1,132 @@
+import doomsday.toolbox as tool
+
+import doomsday.errors as EEEE
+
+from doomsday.const import PHALANX_MONTH
+
 def is_valid_date(date: str) -> bool:
+    '''
+        Check if date = fine (don't ask questions).
+    '''
+    if is_valid_format(date) :
+        split_date: list[int] = tool.parse_date_to_ints(date)
+
+        if (is_valid_year(split_date[0]) and
+        is_valid_month(split_date[1]) and
+        is_valid_day(split_date)) :
+            return True
+
+
+    return False
+
+
+def is_valid_day(date: list[int]) -> bool:
+    '''
+        day = gud ?
+    '''
+    year: int = date[0]
+    month: int = date[1]
+    day: int = date[2]
+
+    is_valid = False
+
+    if day > 0 :
+
+        if month == 2 :
+            is_valid = is_valid_day_case_february(year, day)
+
+        elif month in PHALANX_MONTH() :
+            if day < 32 :
+
+                is_valid = True
+
+        else :
+            if day < 31 :
+
+                is_valid = True
+
+    if not is_valid :
+        EEEE.wrong_day(day, month, tool.is_leap_year(year))
+
+    return is_valid
+
+
+def is_valid_day_case_february(year: int, day: int) -> bool:
+    '''
+        Subshit from is_valid_day() checking for february.
+    '''
+    if tool.is_leap_year(year) :
+        if day < 30 :
+
+            return True
+                
+    else :
+        if day < 29 :
+                     
+            return True
+    
+    return False
+
+
+def is_valid_month(month: int) -> bool:
+    '''
+        is month valid
+    '''
+    if month < 1 or month > 12 :
+        EEEE.wrong_month(month)
+
+        return False
+
     return True
+
+
+def is_valid_year(year: int) -> bool:
+    '''
+        Does the year fulfill is role correctly in our requirements ?
+    '''
+
+    if  year < 1583 :
+        EEEE.wrong_year(year)
+
+        return False
+
+    return True
+
+
+def is_valid_format(date: str) -> bool:
+    '''
+        Says if the date format is fine.
+    '''
+
+    split_date = tool.split_date(date)
+
+    if len(split_date) == 3 :
+
+        return check_year_month_day(split_date)
+        
+    else :
+        EEEE.wrong_format()
+
+        return False
+    
+
+def check_year_month_day(split_date: list[str]) -> bool :
+    '''
+        Subshit from is_valid_format() helping to tell date format validity for
+        each date part individually.
+    '''
+    if (len(split_date[0]) == 0 or                            
+        len(split_date[1]) < 1 and len(split_date[1]) > 2 or 
+        len(split_date[2]) < 1 and len(split_date[2]) > 2 ):
+            
+            EEEE.wrong_format()
+
+            return False
+    
+    for element in split_date :
+        if (not element.isnumeric()) :
+            EEEE.not_numeric_value(element)
+
+            return False
+
+    return True
\ No newline at end of file
diff --git a/doomsday/errors.py b/doomsday/errors.py
new file mode 100644
index 0000000..b70811d
--- /dev/null
+++ b/doomsday/errors.py
@@ -0,0 +1,56 @@
+from doomsday.const import PHALANX_MONTH
+########################### Date validity ######################################
+
+def wrong_format() :
+    print("******* Error Format *******")
+    print("Date Format not valid. Follow this case : YYYY-mm-dd")
+    print("****************************")
+
+
+def not_numeric_value(value: str) :
+    print("******* Error Format *******")
+    print("One of your value is not numerical :")
+    print(f"--> {value} <--")
+    print("Try again with only numerical values.")
+    print("****************************")
+
+
+def wrong_month(value: int) :
+    print("******* Error Value *******")
+    print(f"You entered {value} as month.")
+    print("The month must be between 01 and 12.")
+    print("***************************")
+
+
+def wrong_year(value: int) :
+    print("******* Error Value *******")
+    print(f"You entered {value} as year.")
+    print("The year must be past 1583.")
+    print("***************************")
+
+
+def wrong_day(day: int, month: int, is_leap: bool) :
+    print("******* Error Value *******")
+    print(f"For month number {month}, you entered {day} as day.")
+    
+    if month == 2 :
+        wrong_day_case_february(day, month, is_leap)
+
+    elif month in PHALANX_MONTH() :
+        print("In your case, the day must be between 01 and 31")
+
+    else :
+        print("In your case, the day must be between 01 and 30")
+    print("***************************")
+
+
+def wrong_day_case_february(day: int, month: int, is_leap: bool) :
+        
+    if is_leap :
+        print ("Your year is leap")
+        print("The day must be between 01 and 29.")
+    else :
+        print("Your year is not leap")
+        print("The day must be between 01 and 28.")
+
+################################################################################
\ No newline at end of file
diff --git a/doomsday/toolbox.py b/doomsday/toolbox.py
new file mode 100644
index 0000000..3c310bf
--- /dev/null
+++ b/doomsday/toolbox.py
@@ -0,0 +1,57 @@
+def is_leap_year(year: int) -> bool:
+    '''
+        Check if the year is a leap year.
+    '''
+    if year % 100 == 0:
+        if year % 400 == 0:
+
+            return True
+        
+    elif year % 4 == 0:
+
+        return True
+    
+    return False
+
+
+def parse_date_to_ints(date: str) -> list[int] :
+    '''
+        Get a list of each individual parts of the date.
+    '''
+    return date_elements_to_ints(split_date(date))
+
+
+def date_elements_to_ints(split_date: list[str]) -> list[int] :
+    '''
+        Return the int side of the force. You'd ask me why did I made two 
+        different functions to get the same result. 
+        AHAHAHAAAAAHHH, behold my tremendous genius (and -1 on my score) !!!
+    '''
+
+    split_date_int: list[int] = []
+
+    for date_part in split_date :
+        split_date_int.append(int(date_part))
+    
+    return split_date_int
+
+
+def split_date(date: str) -> list[str]:
+    '''
+        Give an array containing each elements of the date.
+        -> if not a date, return an array containing only the string
+    '''
+
+    if "-" in date :
+        return date.split("-")
+                          
+    elif "." in date :
+        return date.split(".")
+                          
+    elif "/" in date :
+        return date.split("/")
+    
+    elif "_" in date :
+        return date.split("_")
+    
+    return date.split()
\ No newline at end of file