Can anyone tell me why there is 'an' before the days? Why cant it be just 'ar gcéadaoin' wont it be-on the wednesday-
This one would better be translated as "On Wednesdays or on Mondays" with a habitual aspect. To use a day adverbially, you need Dé. But as for why, it's because the day of the week is a definite noun.
It’s similar to why “in the morning” in English requires its “the” — it refers to a particular defined period of time.