Here are the dates we know for sure and dates from years past for those we do not know:
UCB: April 19, 2019
UCSB: March 19, 2019; April 1, 2019; April 15, 2019; April 30, 2019.
UCSC: March 17-April 30, 2019
UCLA:
2018: Wednesday, April 25, 5:10p
2017: Wednesday, April 26, 5p
2016: Friday, April 22, 5p
UCI:
2018: TAG admittances, March 21 (slow roll started April 6)
2017: April 19 at 3p; big drop April 21. (TAG/Honors-to-Honors March 14)
2016: Started Thursday, April 14, with a large drop on Friday, April 22. Regents acceptance first week of March.
UCSD:
2018: Thurs, April 26, 5:40p
2017: Drop March 10; Main drop: April 21 and April 24
2016: Friday, March 11 early drop, then a large gap. Main drop on Friday, April 22
UCD:
2018: Fri, April 20, 3p
2017: April 21, starting at 3p
2016: Friday, April 22, 2p
UCM:
2018: Tues, March 20
2017: Started in April (rolling)
2016: Friday, March 4 (rolling)
UCR:
2018: Mon, March 5 (rolling)
2017: March 14 – May 4 (rolling)
2016: started on Tuesday, March 1 – April (rolling)
Sorry if I missed any…
Source for past dates: http://■■■■■■■■■■■■■■■■■■■■■/2018/03/2018-uc-transfer-admission-notification-dates/