Some people claim, with certainty, that Purim will be the only Yom Tov after Moshiach comes.
But is it really so simple? No.
It is a matter of serious disagreement. See also here.
Good Shabbos, and a freilichen Purim.
P.S. Rabbi Yair Hoffman chimes in, cites בני יששכר and דברי יואל as agreeing that other ימים טובים will continue.