I have worked for three years on a particular Scrum project where we always started the sprint on a Wednesday and ended on a Tuesday. It worked very well and we never had a problem.
I have recently moved to another project where the sprint starts on a Monday and ends on a Friday, and for the first time it made me think about whether it is a good idea to start on a Monday.
Unfortunately, I can see more disadvantages than advantages.
If you are practicing Scrum where part of the team is based in the United Kingdom and the rest is elsewhere, the U.K. has at least four Mondays a year that are always holidays. This is a problem, because if your sprint starts on Monday, the UK-based team members will not be able to participate in the sprint planning meeting -- which is not ideal for any Scrum team.
Some holidays also fall on a Friday (Good Friday, for example), which means that possibly ending the sprint on a Friday is not a good idea. And remember that Friday is a nonworking day in the Middle East region.
My experience of team members based in India and others offshore is that many team members leave early on Friday to go back to their home cities for the weekend (they sometimes travel to other states and need to leave early). As we know from Murphy's law, if anything can go wrong, it will go wrong on the last day, when half of your team planned to leave early for the weekend.
Friday being the last day of the week means that people are usually tired and not so able to concentrate to finish the sprint (they just want to go home for the weekend).
Starting the sprint on Monday will tempt teams to work over the weekend to finish stuff that was not finished on Friday. In certain circumstances this can be acceptable, but in the long run, it is not good for any team to know that they have the weekend as a buffer to complete the sprint before the code snapshot on Monday morning.
If you start a sprint on a Monday, the only benefit I can think of is that the team had the weekend to relax and therefore would be in a good position to concentrate and commit to a new sprint.
I am convinced, however, that the disadvantages outweigh the benefits. I personally feel that the best day to start a sprint is indeed the Wednesday, where concentration levels are usually very high.