A day takes na or no - na segunda-feira for "on Monday", às segundas for "on Mondays". Months take em, and a date is em plus the number: a oito de abril. Days and months are lower case, and the week starts at segunda-feira, which is Monday, not the second day.
Examples
- Tenho aula na quarta-feira.
- I have a class on Wednesday.
- Aos sábados não trabalho.
- On Saturdays I don't work.
- O meu aniversário é a oito de abril.
- My birthday is on the eighth of April.
The mistake to avoid
Segunda-feira is Monday. Counting from segunda as "second" puts every weekday one day out.
Where this fits in
Numbers, days, time, Portuguese A1: Count, talk about days and months, give the time, and handle numbers in real situations.