TL; DR: Wednesday.
Note: I'm assuming one-week sprints in this article.
When I started thinking about this topic, I focused too much on the best day to have the retrospective and planning meetings. It turned out to be a wrong approach. The most important day of a 1-week sprint is the last day of the sprint. You need to make sure to finish all features, integrate them together, test them and deploy them to production. In order to have a successful sprint, the team needs to make sure the most effective day of week will be the last day of the sprint.
Some companies prefer Mondays to kick-off new sprints. I don't blame them. It aligns very nicely with how people think and structure their working weeks. The idea is you finish everything on Friday and enjoy the weekend with a fresh mind. Once you come back on Monday, you can pick up new topics with a fresh pair of eyes.
Not so fast!
I personally believe (and some research supports it), that Fridays are the least productive days. Meaning, the day when you need to bring all the goodies and focus, you're actually the least likely to be as productive as you need to be.
The natural response might be. Ok, let's push the kick-off to Tuesdays then. Meaning that the most productive day should be Monday. While it may turn out to work slightly better, Mondays are usually catching-up days, how-was-your-weekend days and remembering-what-i-didnt-finish-on-friday days. Having a two-days leisure time before the most important day of the sprint might not be a good idea:
- People might experience some anxiety over the weekend about the work they need to finish on Monday.
- The two day context switch might take its toll on remembering all the details one should finish.
What is the answer then?
Tuesdays are the most productive days of the week since the 1987 according to this. Having the Tuesday as the productivity peak, the natural choice to kick-off the new sprint should be Wednesday. I have worked a couple of years with Thursdays as the first day of the sprint and it worked great for me and for the team.
You start diving into the topics and tickets on Friday. Have a relaxing weekend with no stress. You already know what you need to finish next week, so there is a low-priority thread in your head that figures out some details about the upcoming work and reveals important questions to ask. Then as the week progresses, it builds the productivity slowly over the Monday and Tuesday until it reaches Wednesday. The most important day of the sprint.
When do you start your sprints? How does it work for you?