Some schools already have dates on their websites.
UCSD - April 2 - Triton Day
UC Davis - April 9 - Aggie Day (formerly Decision Day) - They are planning on both in person and virtual.
If you can’t make that, Picnic Day - April 23 - is a big open house for the entire community. They are planning on doing an in person Picnic Day this year. If you plan on attending Picnic Day, book hotels now because they sell out quickly for that weekend.
UCSB - April 9 - Open House
UCR - April 9 - Highlander Day. Blog update noting that it is an in-person, on campus event.
UCLA - April 9 - Bruin Day
UCI - April 16 - Open House
UCSC - I don’t think UCSC has a formal open house but, in the past, they have offered tours and admitted student sessions on weekends in April.
UCM - April 23 - Bobcat Day
UCB - April 23 - Cal Day news. Engineering Cal Day may be followed by a week of virtual activities
Not a UC but many students apply to SLO as well. SLO’s open house will be April 7-9. That will be a busy weekend for visiting colleges.