Is Week a collective noun or common noun?

The noun 'week' is a common noun, a general word for a seven day period; a word for an week. A common noun is capitalized only when it's the first word in a sentence.

A collective noun is a noun used to group people or things in a descriptive or fanciful way. The noun 'week' is not a standard collective noun. However, collective nouns are an informal part of language; any noun that suits the context can function as a collective noun, for example, a week of rain or a week of fun.