Overwatch Hero rotation date varies week by week. Rotation should happen every Thursday when normal patches go up, at least every two weeks.
The first rotation happened March 5, seeing Hanzo, Mei, Orisa and Baptist banned for competitive play. The second rotation occurred one week later on March 12, putting Reinhardt, Reaper, Ana and Moira out. If there isn't a rotation for the week of March 19, then there will surely be one on March 26.
Overwatch Hero Rotation Date: When Do Heroes Rotate in Competitive Play?
According to Blizzard, no Hero will be banned for more than two weeks. The official announcements are not clear whether some Heroes will see two week bans while others in their pool become playable. Heroes that have significant buffs or nerfs will not be eligible to be rotated out.
Hero Pools are an experiment for Overwatch Season 21, with Blizzard stating that the system might be scrapped for future seasons.
The Overwatch League will also be observing Hero Pools, but will operate at their own discretion. OWL will look at the last two weeks of play, determine the Heroes that see a play-rate of over 10%. From those Heroes, OWL will randomly ban two damage Heroes, one tank Hero and one support Hero. They will ignore pool restrictions for the midseason tournament, play-in tournament, playoffs or Grand Finals.
UPDATE (4/13 10:45 a.m. ET): Blizzard announced a new Hero Rotation date with hero bans being announced Sunday and implemented Monday.