"It opens on Monday."
Translation:Den åbner på mandag.
Best reason I can think of is because it is. Like in English we would say "on Monday" but "in a couple of days", it's just the preposition that goes with specific days of the week. i is only really used for months or unspecific times of the day (i dag, i aften, i april). Also it could be because "i mandags(/tirsdags/onsdags osv.)" means "last Monday(/Tuesday/Wednesday etc.)"