The book states that McGonagall has been watching the Dursleys from early TUESDAY MORNING. Yes, October 31 1981 was on a SATURDAY, but i believe it's not stated how many days have passed since. After looking into it more, i like to think that Hagrid had taken Harry to a safehouse near Bristol where he waited for further instructions from Dumbledore (which took a day or two) and delivered Harry on TUESDAY NIGHT.
This gives it enough time for Voldemort's death to be confirmed and for it to spread around the wizarding community. Now, there was probably no classes on that day or around that time due to the war, or they possibly cancelled classes for a few days to celebrate Voldemort's downfall.
On the other hand...
"Perhaps people have been celebrating Bonfire Night early — it’s not until next week, folks! But I can promise a wet night tonight.”
Bonfire night is always on the 5th of November which would have been two days after Tuesday, (on Thursday) and not "next week". "Next week" would have applied if it was a Sunday night instead, (as the Dursleys hear the news in the evening they leave Harry at the door) but even then that's barely "next week". Or maybe that's supposed to be on Halloween night? I'm completely lost now