Reasoning-please explain the first two steps.(p:179,exp)

see the attachment

But in case I got to know some other methods (if easier than this), I will surely reply you

for computational formula , see the following attachments

This is the only one way that I know....count the number of odd days.... start with breaking ur years in 400 yrs, then break the left in 100,200 or 300 years, then the left less th...

Thank you sir.