I want to write \"We are closed Sundays and Bank Holidays\"

Do I need an apostrophe on the Sundays & Bank Holidays?

No. They are simply plurals.
One Sunday.
All the Sundays in a month.

Hmm. I couldn't EDIT my response. Word processor must be playing up.

Here is my EDIT, after an afterthought: lol (I can't get smileys to work for me either!)

No. They are simply plurals.
One Sunday.
Two Sundays in a row.
All the Sundays in a year.


grammAR.

Apostrophes are to be used only in the following situations:
1) To replace missing letters (as in contractions).
2) To indicate possessives (as in Linda's book).

Apostrophes should never be used to indicate plurals.
Never, ever.




Also, as Bazza indicated, it's "grammar" not "grammer".

No because we do not use apostrophise in plurals. And the spelling of grammar is incorrect, it should be grammar instead of grammer.