Sunday to Thursday to avoid the crowds.
Vegas is traditionally least crowded between November 28 ish and December 22nd ish. Hotels are extremely cheap during this time. But a lot of restaurants and major shows do take rotational long breaks during this time as well (usually from one week to two weeks long), some shows shut down completely during this time, so plan accordingly.
November (right before Thanksgiving) is the best time to go to Las Vegas. There’s almost no one there and hotel prices are cheap.
Went to Vegas right after Labor Day. Still hot, but not crowded at all and all the hotel rates were super cheap! Great time to go!