Since the Ubuntu devs decided to drop the alpha milestones from their development cycle a few years ago, they had to find a way for the community to test upcoming releases and report bugs.
So they come up with the Ubuntu Testing Week initiative, which is optional for the Ubuntu flavors. During the six-month development cycle of a new Ubuntu release, there are two or three Ubuntu Testing Weeks taking place at monthly intervals. For the Ubuntu 21.04 (Hirsute Hippo) cycle, all flavors (official and unofficial) will be participating in the last Ubuntu Testing Week from April 1st to April 7th.