The reason one doesn’t say tachnun on Tisha b’av is because it’s called (or will be) a moed, if so why aren’t we saying tachnun on Sunday if the yom tov won’t be nidcheh?
Publicly we don’t differentiate in order not to take it lightly.
when do the halachos of mourning end?
Meat is not eaten still Sunday night but can be eaten Monday so the mourning period ends the first thing Monday morning.
I looked it up in the dirshu Mishnah brurah, Yunis hamoshiach when Tisha B’Av falls out on shabbos the yom tov will be a nidcheh too
Either that or I heard in the name of rabbi yaeger that it’s like a beis aveil where there is no tachnun
It’s more than a beis avel since we are all aveilim. Some say tiskabel in a beis avel but not on tisha b’ov.