pyFindNamedDaysForYear

# Find all occurrences of a specified day of the week, for any year from 1901 to 2099
def find_days(yyyy, sday):    # yyyy is the year, sday is the alphabetic day e.g. "Mon"
    sdtab = ["mon", "tue", "wed", "thu", "fri", "sat", "sun"]
    if sday.lower() not in sdtab or yyyy < 1901 or yyyy > 2099:
        print("Invalid day or year")
        return False
    da = sdtab.index(sday.lower())  # convert alphabetic day to index 0 to 6 (mon to sun)
    yd = yyyy - 1901                # get year offset from 1901
    leaps = yd // 4                 # compute number of leap years that have occurred since 1901
    # 01-01-1901 = Monday. 01-01-1902 = Tuesday etc, but we skip an extra day each leap year
    yd = (yd + leaps) % 7           # Compute index to first day of the year in question
    dd = ((da - yd) + 7) % 7        # Find the 1st date in January for the day of week required

    days = [0, 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31]
    if not yyyy & 3:            # We are only dealing with 1901 to 2099 so this simple leap test is OK
        days[2] = 29            # Modifiy days table for February if it's a leap year
    mm = 1                      # Start a loop with month starting at 1
    while mm < 13:              # Loop until we go past December
        if dd > 0:
            print(sday, yyyy, mm, dd)
        dd += 7                 # Bump day by 7
        if dd > days[mm]:       # Check for beyond month end and move into next month
            dd -= days[mm]
            mm += 1

find_days(2021, "Sun")
find_days(1942, "Sun")
find_days(1947, "Sat")

# Samples from results:-
# Sun 2021 3 28
# Sun 1942 7 19
# Sat 1947 3 1