From Redditor u/FantaLemon11:
It's not a huge plot hole, but it's such a convenient coincidence that they always seem to go back to Hogwarts on the 1st of September and yet the next day is always a Monday.
From Redditor u/blastedt:
Harry was born in 1980 so he begins schooling in the 1991 school year.
1991: September 2nd is a Monday.
96: Saturday. They do indeed head out to classes that morning. Book is literally unplayable.
And of course there was no '97 school year because they went camping instead