Like with many answers to questions relating to Scrum the answer is “there is no right answer”. Numerous factors can dictate what is best for a team but this will change from team to team.
In my personal experience I’ve found that starting on a Wednesday is best. Why? Well first of all being based in the UK there are a few bank holidays on Monday through the year and I find that starting on a Monday and having to shift quite a few around is far from ideal. I’ve also found that devs (and humans in general!) don’t like Monday mornings and sprint planning is a vital part of the SCRUM process and have found that not having team members fully engaged (regardless of the date and time you do it) is a recipe for a sprint to not succeed.