The UCs that are on the quarter system (all except for Berkeley and Merced) start on the same day.
Quarter system schools - Quarter begins on September 28 but the first day of instruction is October 1.
UCLA https://www.registrar.ucla.edu/Calendars/Annual-Academic-Calendar
UCD https://www.ucdavis.edu/calendar/academic/
UCSB https://www.ucdavis.edu/calendar/academic/
UCI https://www.reg.uci.edu/calendars/quarterly/2020-2021/quarterly20-21.html
UCSD https://blink.ucsd.edu/instructors/resources/academic/calendars/2020.html
UCSC https://registrar.ucsc.edu/calendar/2020-21calendar-two-pages.pdf
UCR https://registrar.ucr.edu/calendar#first_day_of_class
Semester schools - Semester begins on August 19 but instruction begins on August 26.
UCB http://guide.berkeley.edu/academic-calendar/
UCM https://registrar.ucmerced.edu/schedules/academic-calendar/academic-calendar-2020-2021