that's pretty much night float, although i know it's probably not called that. UPMC has a similar system (i think their residents come in at 3pm).
Not a night float, as we only take these calls a little less than once a week. While the hours may be about the same, now instead of a pre- and post call day every time you have call, most of pre and post call days are the same thing (except the beginning and end of your string of nights). So you lose the time off surrounding calls.
We have a discussed a true night float, that is say Sunday through Friday nights you do the 4-7 gig. Everyone one dose a week at at time. The upside is you have one more resident in the OR (in our system now the post and pre-call residents aren't around), the down side is you lose the pre-post call days surrounding you calls, cause they're sandwiched together, you're trying to catch up on sleep. Most of us are opposed to the idea.
Of note, at UW and Harborview we have two residents main OR resident on call each night (childrens and VA only one), #2 resident is almost always a CA-1. At UW this OR2 call is technically a 24 call, but most of the time they are just on home call up to 4 pm. Weekends at the UW the OR2 is home call for all 24 hours, usually they come for the morning, help get cases done. The Sunday OR2 doesn't get monday off, but they are not suposed to do cases past 8 pm on Sundays (which doesn't happen much). If a second resident is need the cardiac resident (who is on home call for cardiac cases on a call schedule) comes in to take over, as they have their post call days off.
Getting back to the original question, everyone does things a little different, and it can vary between hospitals and rotations. You just have to ask the resident at the programs you are curious about. They'll all know which OR months have the worst call schedule. Consider their late schedules too. Most programs probably assign some people to stay late each evening. Often being on the late schedule is worse than being on call, because you generally still have to come back the next day.