Or “Why Don’t We Just End the Iteration of Friday, and start again on Monday?”
For many, it would be logical start and end an Iteration (Sprint) around week boundaries. In general this should be discouraged as:
Holidays and vacations: Most long weekends and other holidays happen on a Monday or a Friday. If we were to have events like Iteration Planning, and the Retrospective scheduled for a Monday, then we would end up rescheduling a lot of these. We might even feel we should shorten or lengthen the timebox, the iteration, which is poor practice.
This situation is made worse when you incorporate significant timezone differences within a Team. For example, planning for an Iteration to finish on a Friday (US time) means that someone is staying up late in Australia or India, and that on a Friday night. While this might be possible in the short term, it is not sustainable in the long term.
To avoid the “billiard ball” pattern: If you set up Iterations so they start and end around the weekend, it reduces the smooth movement from one Iteration to the next. The Monday becomes “oh we have to get started again” and in the end reduces the smooth flow of value. The metaphor comes from a picture in your head: the weekend becomes like a one billiard ball smacking into another.
To avoid burnout: If you start and end around the weekend, people (i.e. intelligent, motivated knowledge workers) will start to treat the weekend like a buffer. Increasingly people will say to themselves “I'll just finish this on the weekend” which will result in a pace of development that is not sustainable. This happens less with a midweek start.
It should be noted that some of this discussion is moot if you are on a Team which is part of a larger team-of-teams (Train) structure. In these cases you will need to align your cadence with that of the larger structure to ensure you can synchronize your activities with other members of the Train. That being said, the same thinking applies to Trains as Teams - we should avoid stating and stopping train event on weekend boundaries.