"We close the cupboard every night."
Translation:Dyn ni'n cau'r cwpwrdd bob nos.
So why wouldn't the 'bob' come °after° nos, since it is describing which night?
Because pob ('each, every') is one of a small class of words which come before the noun they are describing. pob does not cause mutation of a following noun.
(pob nos takes an initial mutation here because it is acting as an adverb of time.)