I took the VSP in 2007 and have some experience with the mandatory muster. They basically wanted everyone to attend once a year at your local Air Force Base. I blew it off the first few times (not completely on purpose, my forwarding address was different than my new home address) I then received some certified mail basically threatening to bring me back to active duty as punishment for some form of dereliction of duty. I sent a letter apologizing and got in contact with someone in charge of the IRR. It wasn't quite as serious as they made it out to be, but they wanted me to attend the next muster. The muster seems to just be some sort of head count and make sure they still have your current contact info. They sat us in a big auditorium and covered some good information about veteran benefits, VA benefits, etc. It was actually fairly informative and was glad I went. Not sure where you're living, but they usually give you a few other options/locations if the date they send you doesn't work with your schedule. I think they gave me 3 different options. So if it doesn't work with your schedule you can try calling and trying for a different date. You do get some pay for it, but it'll basically just pay your gas bill to drive there.