seq.py

#!/usr/bin/env python

months =              "Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec".split(" ") 
days_normal = map(int, "31  28  31  30  31  30  31  31  30  31  30  31".split("  "))
days_leap   = map(int, "31  29  31  30  31  30  31  31  30  31  30  31".split("  "))

assert len(months) == 12
assert len(days_normal) == 12
assert len(days_normal) == 12
assert sum(days_normal) == 365
assert sum(days_leap) == 366

for ymod400 in range(0, 400):  # [0, 1, ..., 399]
    for monthnum, monthname in enumerate(months):
        isleap = ymod400 % 4 == 0 and (ymod400 % 100 != 0 or ymod400 % 400 == 0)
        num_days = (days_leap if isleap else days_normal)[monthnum]
        for daynum in range(1, num_days + 1):  # [1, 2, ..., num_days]
            print("%s %d in a year $y mod 400 = %d$" % (monthname, daynum, ymod400))