Yesterday, someone in the comments said Professor Lupin looked exhausted because he'd just transformed into a werewolf.
I thought about it more carefully, and suddenly realized there was a problem.
September 1, 1993, fell on the fifteenth day of the seventh lunar month. Professor Lupin should have transformed into a werewolf that day, so how could he ride the Hogwarts Express and attend the opening feast?
Someone might say he only transformed when the moon came out that night.
But I checked, and Christmas Day, December 25, 1993, fell on the thirteenth day of the eleventh lunar month. By then, Professor Lupin was already ill and couldn't even attend the Christmas feast.
The books also mention several times that Professor Lupin had Professor Snape cover his classes, and each time he was away for several days.
That suggests there should already have been signs before the fifteenth day of the lunar month, forcing Professor Lupin to rest.
Wolfsbane Potion could help werewolves retain their reason, but it couldn't turn them back into human form.
Based on all this, a werewolf's transformation might last for some time. At the very least, there were signs beforehand that kept them from appearing in public.
But then the author worked out another date.
According to the original story, the second-to-last exam was the Defense Against the Dark Arts exam, which Professor Lupin supervised himself, and it took place on a Thursday.
After the Divination exam ended that afternoon, the final exams were over, and Professor Lupin transformed into a werewolf that night.
I checked the calendar and concluded that the day must have been June 23, 1994, which happened to fall on the fifteenth day of the fifth lunar month.
After all, none of the other Thursdays in June coincided with a full moon,