Rainbow Six: Siege’s closed alpha on PC will start on the 7th Of April at 10 AM Eastern, ending on the 13th of April at 10 AM Eastern. (more info)
Also, closed alpha invites are already being sent out to gamers, and once some one has access, it will be already on their Uplay account to start pre-loading. The game’s size is around 9GB to download.
For more info, and to sign-up for the closed alpha, go here.