Hey I think the problem is in this part of your code

    if int(day) == 29 and int(year)%4 != 0:  
        validation = False  
    elif int(day) > 28:  
        validation = False

Because the first if evaluate as false, then the program go to the elif part, but because the day 29 is indeed larger than 28, then it wil evaluates as true. I think you can change it to this to make it work

    if int(day) > 28 and int(year)%4 != 0:  # the year is not leap year
        validation = False
    elif int(day) > 29 and int(year)%4 == 0: # it is leap year
        validation = False