The Halloween Feast and the Mountain Troll

The feast is interrupted by a Mountain Troll who almost kills Hermione Granger; she is saved by Harry and Ron, after which they become friends.

Meanwhile, Quirrell, who let the troll in, heads for the Philosopher’s Stone hidden beneath the trapdoor in the third floor corridor – but is stopped by Snape.

Timeline Notes

We have already seen that we can't trust the "real" calendar to give us the days of the week when we know the date, but just for interest sake, Halloween in 1991 was on a Thursday. The Gryffindor first years had Charms that morning and learned the Wingardium Leviosa spell.