French has no word for "on" with a day: lundi alone means "on Monday", while le lundi means "on Mondays", every week. Months take en, and a date is le plus the plain number - le cinq mai. Days and months are lower case.
Examples
- J'ai un cours mercredi.
- I have a class on Wednesday.
- Le samedi, je ne travaille pas.
- On Saturdays I don't work.
- Mon anniversaire est le huit avril.
- My birthday is on the eighth of April.
The mistake to avoid
Only the first of the month uses an ordinal: le premier mai, but le deux mai.
Where this fits in
Numbers, days, time, French A1: Count, talk about days and months, give the time, and handle numbers in real situations.