There's a lot wrong with this idea.
First, everything from Sprint Planning through the Sprint Retrospective is part of a Sprint. Also, a new Sprint starts immediately when a Sprint ends (although, in my experiences, I've typically seen the Sprint Review and Sprint Retrospective in the afternoon and then Sprint Planning the next morning, with a handful of working hours between them). All time, including weekend and holidays, is part of the current Sprint.
The Sprint Backlog is something that has been agreed upon by the Development Team. A Sprint tends to be defined by the Sprint Backlog and the Sprint Goal. During Sprint Planning, the team should have looked at the ordered Product Backlog, determined what items could be done this Sprint, made some kind of plan for getting it done, and then defined objectives that guide the Sprint and help focus the team.
Given this, I would have concerns about just working extra hours without telling anyone (preferably in advance), even if it was on planned Sprint work. If the plan was built around a specific capacity and people work additional hours, that's going to throw off planning for future Sprints unless it's accounted for. There could also be other downstream implications with respect to testing, deployments, change management, customer needs or expectations, and so on.
I would suggest that working outside of working hours is a violation of the principles of Agile Software Development. One of the principles is about sustainable development - the pace that the team is working should be maintainable indefinitely. Working weekends on unplanned work is not sustainable in the long-term.
- Don't work more than you've planned for a Sprint. Even if you are falling behind, work with the rest of the team and the Product Owner to decide what the best thing to do is.
- Don't work outside of the Sprint Backlog. All of your planned capacity and effort should be to realizing the Sprint Goals.
- Keep the underlying principles of Agile in mind. Sustainable pace is necessary. Not only does it prevent you from burning out, but it leads to better estimates and plans.