Whilst not a generally accepted scrum principle (excellent article from Mike Cohn http://www.mountaingoatsoftware.com/blog/gasping-about-the-product-backlog) in my experience as a Scrum Master i have found that Backlog Grooming (or Backlog refinement as it is often called) is a hugely important exercise for a Scrum team.
As Mike Cohn states in the post above the backlog grooming meeting isn’t part of the core Scrum rules nor is it a Generally Accepted Scrum Practice (GASP) but more and more teams are adopting this.
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.