A day takes в plus the accusative - в понедельник. A month takes в plus the prepositional - в мае. A clock time takes в plus the number: в восемь часов. Days and months are written in lower case.
Examples
- В среду у меня урок.
- On Wednesday I have a class.
- В апреле я еду в Вену.
- In April I'm going to Vienna.
- Мы встречаемся в восемь.
- We're meeting at eight.
The mistake to avoid
The same в takes different cases with a day and a month - в понедельник but в мае.
Where this fits in
Numbers, days, time, Russian A1: Count, talk about days and months, give the time, and handle numbers in real situations.