Brute force

This problem is not fascinating, we could start from the first of January 1901 as a Tuesday, and iterate on each month, for example the first day of February 1901 is Tuesday + $31\ \%\ 7$, which is Friday. We could continue by taking leap year into account. But thanks to Python, we can solve this problem with a simpler solution:

From solution1.py:

def counting_sundays():
    res = 0
    for year in range(1901, 2001):
        for month in range(1, 13):
            if datetime.datetime(year, month, 1).weekday() == 6:
                res += 1

    return res