The releases are the things that are few and far between - generally though, a nominated daily-build (based on the pre-determined release schedule) is triaged and tested by QA and engineering for a while before release, and then ... it's out there...
...Unless something goes unexpectedly wrong with the nomination, anyway. That's pretty rare because builds are constantly being made, regressions identified, and new bugs discovered and earmarked as "must-fix" (or whatever) on a daily basis. B&I have a fairly good feel for how things are going at any given time.
It's really just timing. If you can squeeze another fix in before the cutoff deadline for the nominated build, you're in. If not, you wait until the next one, which can be a while...