Short answer: July 4th.
It's actually two issues, opening day for the resorts and opening day for the theme parks. I see a phased-in opening schedule vs a shotgun start.
I predict resorts open on a rolling schedule, say one resort every two weeks until the total general U.S. population can travel freely. Although staffing and operations will be challenging, it's nothing that Disney can't handle. Rather, the critical factor will be what their customer based can provide in terms of patronage.
Theme parks might be available for a memorable opening by July 4th, starting with MK. A patriotic goal for a truly American institution. Two, maybe three, resorts will open by that date to feed the beast. FW will open by then, maybe even the first to open since it's an easier challenge to conquer.