how does this calendar work?

hi, this is a template i got online.

i have used it in different situations and all.

What i am trying to figure out is how does the calendar part work?

what makes the numbered dates start where they do?

I got alot of the other stuff figured out and have added and taken away stuff

i just cant figure out how that calendar dates work. i see the conditional rules. ive tried to recreate it on a new sheet and wasn't able to get it to work.

thank you all

calendar.jpg