What is the function of "Monday?"

What is the function of "Monday?" Is it a direct object of starts or an adverb?

Mask mandate starts Monday.

Neither. It is a noun, but it is not being used as a direct object, but as a time adjunct. This is a function that can be carried out by phrases headed by words belonging to various parts of speech (adverbs, but also prepositions and nouns). The adjunct function can also be called “adverbial”, which is distinct from the part of speech “adverb”.

In a question with corresponding form, “Monday” would be replaced by “when”, which is the interrogative word used for time adjuncts: “When does the mask mandate start?”

You can tell it is a noun because it can be preceded by a determiner or adjective: “starts this Monday” or “starts next Monday” both work.

You can tell it is not an object of the verb as it cannot be made into the subject of a passive clause: *“Monday is started by the mask mandate” is only grammatical when it has a different meaning.