אלה הדברים אשר דבר משה…(דברים א:א)

The Passuk lists seven aveiros – sins that the Yidden committed. To rectify these sins, we celebrate seven days of Pesach, seven days of Sukkos, seven days of Shavuos (including the ימי תשלומים – the additional days during the times of the Beis Hamikdash when one was able to bring a korban). And when Moshiach comes, we will celebrate 21 days from י’’ז תמוז until ט׳ אב – parallel to the 21 days of Yomim Tovim that we celebrate in Galus.