well, .... don't quote me on this but .... my experience with the "park closed at sunset" type signs, are for purposes of keeping out late night teen keg parties. Or to keep out homeless who think they can camp out there, etc.... But otherwise, it wouldn't be unusual to see someone walking their dog, taking a romantic night stroll with their honey, etc....
But assuming you truly can't do it at night (ie.: assuming someone there "really cares"), then how about crack of dawn Sat. Or Sun AM's ? Or on federal holidays when city staffs are off for the day. Or at times when night-fall isn't till 9pm, such that any rush hour traffic, and/or city park staff are all done for the day. Ie.: hunt 8pm to 9pm so that you're not (gasp) violating the "sunset" signage.