Hello, Noeima!
Here's the link to the most recent
Magic Kingdom Park March calendar. This calendar notes that Magic Kingdom Park will open at 9:00 a.m. on Thursday, March 1st. If you look at the February calendar, however, it shows that MK will be open on February 29th, 2012, from 6 a.m. to 6 a.m. To me, this seems as if they'll close the park at 6 a.m. on the 1st, then open it back up again at the usual opening time of 9:00 a.m.
If, for some reason, you decide to head into MK upon your arrival on the 29th, you'll probably find unique merchandise such as souvenir T-shirts and such, which commemorate your having been in the park on Leap Year Day, 2012. (We were in MK for the 40th birthday celebration, and found a few unique items for that event as well.)
Have a fantastic vacation in Walt Disney World! Send any additional questions that you might have regarding your upcoming vacation; we love to help!