There is no possession in the legal sense, but there is grammatical possession, and the apostrophe is therefore necessary! Thus:
one day's/night's/week's (etc.) accommodation
two days'/nights'/weeks' (etc.) accommodation.
You may possibly be getting these determinative nouns confused with (invariable) compound adjectives such as 'three-day' in a three-day trip. Sometimes, of course, both constructions are possible, so that two weeks' vacation is essentially the same in meaning as a two-week vacation.