noun (often initial capital letter) (usually used with a singular verb) Chiefly British.
Examples from the Web for mattins
We make it known we shall be pleased to see them after Mattins and Evensong.
They don't say Mattins, which I'm inclined to think rather slack.The Altar Steps|Compton MacKenzie
We have no right to complain for a moment of what some people call the monotony of Mattins and Evensong for ourselves.
These seven offices were condensed in 1519 into two, our present Mattins and Evensong.The Leper in England: with some account of English lazar-houses|Robert Charles Hope