take a seerees av steps
1. selekt a start yeer such as 0
2. find out dae-av-week for 0 Jan 0
3. add all daes from 0 Jan 0 tu date yu want
4. divide this number bi 7 & get leftover
5. add leftover tu dae-av-week av 0 Jan 0
if > 7, divide bi 7 & get leftover
wen me duit, me get anser daes=1, 2,...7
but many koders on the internet yuze dae numbers=0, 1, 2...6