Actually, they announced that it was moving to Thursdays...and then, a few hours later, announced that it would be staying on Tuesdays. Here's the logic: (going from memory after reading a WSJ article on the subject)
NBC is re-instituting their "Must See TV" slogan.
Thursdays are generally considered a good TV night--great time to advertise weekend activities, such as new movies.
However, NBC has Thursday Night Football, which would preempt regular programming for 5 weeks. In addition, the regular programming would be up against TNF on CBS an additional 5 weeks.
So, they decided to stick with Tuesdays. Not sure why nobody pointed all this out to them, months in advance, but whatever. I'd rather see them stick with one night, any night, although in this day and age of DVR-ing, it's a lot easier to catch a show that you might have missed due to moving/preemption. I know over on CBS, you never know when an episode of Big Bang Theory is going to pop up.