Learn Italian: In quella basilica si celebra il patrono il lunedì. - In that basilica, the patron is celebrated on Monday.

This is the passive si construction. While è celebrato is grammatically correct for "is celebrated," Italian strongly prefers using si + a 3rd person verb for general, customary actions, like a recurring festival or tradition.

Does il lunedì mean "on Monday" or "every Monday"?

Generally, adding a definite article before a day of the week means the action is recurring, so il lunedì usually means "on Mondays" or "every Monday." In the context of a yearly festival, it means the event always takes place on "the Monday" of that particular celebration. For a single, one-off Monday, you would drop the article and just say lunedì.

Why is the subject (il patrono) placed after the verb?

Italian word order is flexible. With the passive si construction, it is very common to put the subject after the verb, especially when the subject is the focus of the sentence. Il patrono si celebra is also correct, but putting the verb first flows more naturally in this phrasing.

If I wanted to add the word for "holy" (santo), would it shorten to san patrono?