Between Ashrei and Uva L’tzion we say Lamnatzeach. It’s apparently a somber psalm referring to the period before the Beis Hamikdosh was rebuilt, and since it’s sad in nature we don’t say it on festive days such as Rosh Chodesh, Purim, Chanukah etc. but we also don’t say it on Tisha bav, one of the saddest days in our calendar…why?
Sorta related but doesn’t really give an answer: Why is Tishah B’Av considered a holiday?