I didn`t understand this situation(Its about tuples)

Project subject is group of people and in this group "what is the chances 2 people can birthdays is same day" So I wanna give birth this people again and again but in this code(Project designers) Month is a tuple and Designer in this code still giving people birth random months and days but I can`t understand.Because what I know is(Tuples cannot be modified)so how designer still can random months in this code?

import datetime,random

def getBirthdays(numberOfBirthdays):
    """Returns a list of number random date objects for birthdays."""
    birthdays = []
    for i in range(numberOfBirthdays):
        # The year is unimportant for our simulation, as long as all
        # birthdays have the same year.
        startOfYear = datetime.date(2001, 1, 1)

        # Get a random day into the year:
        randomNumberOfDays = datetime.timedelta(random.randint(0, 364))
        birthday = startOfYear + randomNumberOfDays
        birthdays.append(birthday)
    return birthdays

MONTHS = ('Jan', 'Feb', 'Mar', 'Apr', 'May', 'Jun',
          'Jul', 'Aug', 'Sep', 'Oct', 'Nov', 'Dec')

while True:  # Keep asking until the user enters a valid amount.
    print('How many birthdays shall I generate? (Max 100)')
    response = input('> ')
    if response.isdecimal() and (0 < int(response) <= 100):
        numBDays = int(response)
        break  # User has entered a valid amount.
print()

# Generate and display the birthdays:
print('Here are', numBDays, 'birthdays:')
birthdays = getBirthdays(numBDays)
for i, birthday in enumerate(birthdays):
    if i != 0:
        # Display a comma for each birthday after the first birthday.
        print(', ', end='')
    monthName = MONTHS[birthday.month]
    dateText = '{} {}'.format(monthName, birthday.day)
    print(dateText, end='')
print()
print()