grammaticality - Can "Monday", "Tuesday" be plural like "Mondays" or "Tuesdays"?

Which of the following sentences is correct?

1. From now on, the biochemistry lecture classes on Mondays at 10.30 a.m. will be held on Sundays at 11.30 a.m instead.


2. From now on, the biochemistry lecture classes on Monday at 10.30 a.m. will be held on Sunday at 11.30 a.m instead.


3. From now on, the biochemistry lecture class on Monday at 10.30 a.m. will be held on Sunday at 11.30 a.m instead.


4. From now on, the biochemistry lecture class on Mondays at 10.30 a.m. will be held on Sundays at 11.30 a.m instead.