Another core principle of agile methods is transparency. In order to improve, you need to be honest about acknowledging changes in plans, mistakes which cause more work, and misunderstandings that cause you to get things done significantly ahead of schedule.
A decision that teams make independently of the number of hours that they need to plan for is what the boundaries of a sprint should be, particularly with a 1 week sprint. This question is especially relevant if you think that weekend work will be needed. If you are doing 1 week sprints, there are basically 3 choices:
- Have sprints be Monday thru Friday, with a review first thing Monday.
- Have sprints be Friday thru Thursday.
- Have sprints be Tuesday thru Monday
The second option is better; the weekend is part of the sprint boundary, and if the team agrees to work weekends you can measure that. Early in the sprint you may be more optimistic and not expect that to work, and early if you front load the riskier work, then the you might find it more useful to plan to do this work during the week when you can count on everyone being around (assuming that weekend work is done at people's own schedule).
The third option means that the weekend at the end, so you'll know if there is a need for extra work. Typically the work at the end of a sprint involves more defined tasks; you may be wrestling with getting something to work, but you probably had the design discussions already and you've tacked all of the tricky issues at the start. At this point going into a corner on the weekend to work is less likely to adversely affect someone else's work.
While each team should define what "sustainable pace" means, and how best to meet their goals, to get the full value of an agile process, keep any days you expect to work inside the boundaries of the sprint. And when you do your release retrospective, be sure to discuss how well the team is managing its workload.