<s>[INST] I am trying to find the day of the week for a specific date.
I am using the Zeller's Congruence algorithm which says that to find the day of the week for a date in the Gregorian calendar, you can use the following formula:

h = (q + [(13*(m+1))/5] + K + [K/4] + [J/4] - 2*J) mod 7

where
h is the day of the week, with Sunday = 0, Saturday = 6
q is the day of the month (1-31)
m is the month (3 = March, 4 = April, 12 = December, 13 = January, 14 = February)
K is the year of the century (year % 100)
J is the century (year / 100)

However, I am having some difficulties with figuring out how to calculate m for January and February.
According to my notes, for January and February, m is equal to the actual month number minus 1. So January = 13 - 1 = 12 and February = 14 - 1 = 13. However, this makes no sense to me as it swaps the month numbers around.

Could you please explain the logic behind using (actual month number) - 1 for January and February?[/INST]