Zeller's congruence for the day of the week (https://en.wikipedia.org/wiki/Zeller's_congruence) exploits this:

- months are counted as 3, 4, 5, ..., 14, with 13 and 14 being January and February of the following year

- the contribution of the month to the day of the week is floor(2.6 * (m+1)) - the 2.6 comes from the 13 "extra" days (over the approximation 1 month = 4 weeks) in every 5 months.