East bay Regional Parks ....
SECTION 906. - CURFEW. (I/M)
No person other than the house guests of the concessionaire-resident or an employee renting a house from the District, persons fishing in parks designated for nighttime fishing, or person possessing valid written permission to camp, shall enter upon or otherwise remain within the District for any purpose whatsoever between the hours specifically posted as curfew hours at the entrance to the particular District parkland, or where no hours are posted, between the hours of 10:00 p.m. to 5:00 a.m. Those persons possessing a valid permit shall be allowed to remain and use parklands and facilities as specified in their permit, after which time they shall leave the District without any appreciable delay.
And the hours there are from 6:30 AM until 8:00 PM.
To me, the term "curfew hours" can be a little obscure. But, if the common interpretation is that this section refers to being in the park before opening hours, then you would lose this in a contested fight. I suspect that the "curfew hours" are those hours where the park is closed. The notice you received went with the boilerplate default, but the code section explicitly states that the 10 PM to 5 AM hours are only if no other hours are posted.

