A day takes w plus the accusative: w poniedziałek. A month takes w plus the locative: w maju. A date uses the genitive and an ordinal number - piątego maja. Days and months are written in lower case.
Examples
- W środę mam lekcję.
- On Wednesday I have a class.
- W kwietniu jadę do Wiednia.
- In April I'm going to Vienna.
- Spotykamy się o ósmej.
- We're meeting at eight.
The mistake to avoid
The same w takes different cases with a day and with a month - w poniedziałek but w maju.
Where this fits in
Numbers, days, time, Polish A1: Count, talk about days and months, give the time, and handle numbers in real situations.