Not getting any output when using 'in', only getting output when using 'not in'

So I had to write some code for my class that takes a date input and based on the month/day outputs the season. When I use "in" on the 10th line I don’t get any output, not even ‘Invalid’ the only time I get an output is when is use ‘not in’. I’m confused why that happens if anyone could explain it to me I would really appreciate it. Thank you.

input_month = input()
input_day = int(input())

month_winter = ['January', 'February', 'March', 'December']
month_spring = ['March', 'April', 'May', 'June']
month_summer = ['June', 'July', 'August', 'September']
month_autumn = ['September', 'October', 'November', 'December']
month_list = [month_winter, month_spring, month_summer, month_autumn]


def month_check():
    if input_month not in month_list[0:]:
        return which_month()


def which_month():
    if input_month in month_winter[0:]:
        which_dates1()
    elif input_month in month_summer[0:]:
        which_dates2()
    elif input_month in month_spring[0:]:
        which_dates3()
    elif input_month in month_autumn[0:]:
        which_dates4()
    else:
        print('Invalid')


def which_dates1():
    if input_month in month_winter[0:2]:
        if 0 < input_day < 32:
            print('Winter')
        else:
            print('Invalid')
    elif input_month in month_winter[2:3]:
        if 1 <= input_day <= 19:
            print('Winter')
        else:
            print('Invalid')
    elif input_month in month_winter[3:]:
        if 21 <= input_day <= 31:
            print('Winter')
    else:
        print('Invalid')


def which_dates2():
    if input_month in month_spring[0:1]:
        if 20 <= input_day < 32:
            print('Spring')
    elif input_month in month_spring[1:3]:
        if 1 <= input_day <= 31:
            print('Spring')
    elif input_month in month_spring[3:]:
        if 1 <= input_day <= 20:
            print('Spring')
    else:
        print('Invalid')


def which_dates3():
    if input_month in month_summer[0:1]:
        if 21 <= input_day < 32:
            print('Summer')
    elif input_month in month_summer[1:3]:
        if 1 <= input_day < 32:
            print('Summer')
    elif input_month in month_winter[3:]:
        if 1 <= input_day <= 21:
            print('Summer')
    else:
        print('Invalid')


def which_dates4():
    if input_month in month_autumn[0:1]:
        if 22 <= input_day < 32:
            print('Autumn')
    elif input_month in month_autumn[1:3]:
        if 1 <= input_day < 32:
            print('Autumn')
    elif input_month in month_autumn[3:]:
        if 1 <= input_day <= 20:
            print('Autumn')
    else:
        print('Invalid')


month_check()