The movie is showing in theaters now and I've read that there are some spoilers from the manga even if you're caught up to the series.
Is it known when this movie takes place?
The movie takes places some time after the ending of the 4th Season of BnHA, sometime in the winter season (but the action is placed on a tropical island). It is also said that possibly the events that take place only happen in the movie.
From the interview with the movie director:
Nagasaki: "Yes, we did use an element that was originally planned in the manga's last battle and used it towards the end of the movie.[...] Besides, Horikoshi-sensei told us "I'll create an even better ending anyway, so it's all good".
So it's not really an actual spoiler for the manga readers, more that the movie adapted some of Horikoshi's previous ideas for the ending of the manga.