Whether starting a new software project or simply transitioning to agile project management, your team will need to pick a length for upcoming sprints.
Scrum doesn’t state anything about deadlines being communicated. The fact is, business objectives have to be met, and they often come with constraints.
The retro is seems like the first meeting to get cut from the normal sprint cadence. But don't give it up! Everybody benefits, whether they know it or not.