Matins, which may also be written as martins, is a service of morning Christian prayer. Or it may be a night 'office of prayer', but it can also happen at daybreak or in the evening. There, aren't you glad I cleared that up for you?
Matins (or mattins) is both the singular and the plural, so at least that's simple.
You may recall me moaning about poets getting an extra definition for the word bedew (no reason you should remember, but you might). They're at it again with matins as they can use it to describe birdsong. Only in the morning though.