I thought using 'ar an' with days of the week implied an habitual aspect. Shouldn't '...on Mondays' be accepted?
Ar an Luan can mean a specific Monday if used with a verb with a non-habitual aspect, e.g. Feicfidh mé ar an Luan thú (“I’ll see you on (this coming) Monday”). With a habitual aspect verb like imreoimid, “on Mondays” should be accepted.