Our schedule is usually every other Thursday evening for patches or minor version upgrades. Usually one Saturday a quarter for a major release. The thinking for Thursday is that we have Friday to fix anything or rollback and then the weekend if needed, but since they are only minor releases, fixes/rollbacks are usually fairly quick (and rare). Major releases on Saturday's get the affected user community in to test the move to production, allowing part of Saturday/all of Sunday to fix or rollback. It works for us.