Zeller$$s congruence is an algorithm developed by Christian Zeller to calculate the day of the week. The formula is

h $=($q $+26($m$+1)//10+$ k $+$ k$//4+$j$//4+5$j$)\%7$

where

$-$ h is the day of the week $(0$: Saturday, $1$: Sunday, $2$: Monday, $3$: Tuesday, $4$: Wednesday, $5$: Thursday, $6$: Friday$).$

$-$ q is the day of the month.

$-$ m is the month $(3$: March, $4$: April, $...,12$: December$).$ January and February are counted as months $13$ and $14$ of the previous year.

$-$ j is year$//100.$

$-$ k is the year of the century $($i$.$e$.,$ year $\%100).$