Campspots turn over in the morning since the regulations dictate that sites must be vacated by noon, with many groups packed out soon after breakfast.
If arriving late in the day, know that camping just outside of Yosemite for the night can work to your advantage rather than searching for a site in the dark, plus you'll be close to the park entrances for an early start the next day.
The park calms down a little in the winter, but even in the “off season” you'd be surprised how busy Yosemite is year round.
Be flexible with whatever's open and it'll help land a campspot much quicker than being picky about it.
Once you've staked claim on an open spot, head to the campground registration office to officially lock it down ASAP. If you've found an open site at night, then get up early and be first at the registration office (opens at 9am) - do not snooze on this or you'll spend more time standing in line than catchin' up with El Capitan.
If this is at a campground, there's a slim chance of staying as there’s most likely already a hold on it.
If available, the ranger will offer other campsite options that fit your schedule, even if it means having to jump around to a different spot every few days to make it work.
Just think of it as a great way to "tour" the campgrounds in Yosemite.