I originally voted July for optimism purposes, but I'm beginning to coalesce around September. Here is my reasoning:
1) Enhanced UI (that +$600 from the federal government for unemployment) expires ~July 25. For a lot of CMs the park, the combination of this + standard unemployment from California makes it more lucrative to stay at home for many (the equivalent of $26/hr give or take, i'm estimating). It would actually be a disservice for TWDC to recall employees to make less (and is a topic many businesses are struggling with).
2) As that period ends, you can recall your employees in August to make preparations for reopening the park -- they would spend all of August applying signs/stickers/paint, augment rides and shopping, and most importantly.... training.
3) Schools would be back in session at that point, another sticking point for bringing back CMs who have children/childcare issues.
4) Demand would be lower as kids are back in school (as stated above by
@cruisehopeful )
So I guess that's that.... I'd say July for non-Disney vendors at DtD, August for DtD businesses o/o by TWDC, and September for parks.
Not 2021 because I want that dang UBS analyst that everyone and their mama quotes to be wrong