A day takes την or το with no preposition - τη Δευτέρα means "on Monday". A month takes τον or τo with σε, contracted to στον or στο: τον Μάιο or στον Μάιο. Days and months are capitalised in Greek.
Examples
- Την Τετάρτη έχω μάθημα.
- On Wednesday I have a class.
- Τον Απρίλιο πάω στη Βιέννη.
- In April I'm going to Vienna.
- Θα βρεθούμε στις οκτώ.
- We'll meet at eight.
The mistake to avoid
Unlike most European languages, Greek CAPITALISES days and months.
Where this fits in
Numbers, days, time, Greek A1: Count, talk about days and months, give the time, and handle numbers in real situations.