Use am with a day (am Montag), im with a month or a season (im Mai), and um with a clock time (um acht Uhr). Days and months are nouns, so they keep their capital letter.
Examples
- Am Mittwoch habe ich einen Kurs.
- On Wednesday I have a class.
- Im April fahre ich nach Wien.
- In April I'm going to Vienna.
- Wir treffen uns um acht Uhr.
- We're meeting at eight o'clock.
The mistake to avoid
Am, im and um are not interchangeable: am Montag, im Mai, um acht.
Where this fits in
Numbers, days, time, German A1: Count, talk about days and months, give the time, and handle numbers in real situations.