Best day of the week for Vatican?

I was trying to figure this out also and in researching on some other sights I read that the least crowded days are Thursday, Tuesday and Friday (in that order). Monday is the most crowded because a lot of other museums in Rome are closed that day.

thursday is usually the least crowded but the museums are always crowded. You'll find all the rooms packed with people and during the summer and high season you'll be pushed along like cattle on the "forced march". Try to go later in the day after the tour groups have arrived. This will give you a little relief but not a ton. Pre-purchase tickets or a tour through the vatican website to avoid any line that may be outside.

Donna