Hi Priya! Thanks for visiting me again at the Moms Panel!
On December 3, the Magic Kingdom's normal operating hours are 9 a.m. to 7 p.m. At 7 p.m. Mickey's Very Merry Christmas Party begins for those guests who have purchased tickets to that special event. The park attractions operate during the party, but only guests attending the party are permitted to ride. Guests without party tickets will be asked to leave the park at 7 p.m. Some guests may have dining reservations that begin shortly before the park closes, and they will, of course, be allowed to finish enjoying their meals before exiting the park.
If you don't have tickets to Mickey's Very Merry Christmas Party and you aren't planning to buy them, you'll need to move your dining reservation at Tony's Town Square to an earlier time before the official park closing. (You should have been notified when you made your dining reservation that you'd need tickets to the party to enter the Magic Kingdom after closing to dine at Tony's.)
Mickey's Very Merry Christmas Party is a fun event, and if you've never been, consider buying tickets. Party guests are treated to a special holiday parade, snow on Main Street USA, cookies and hot cocoa, special character meets and performances.
Priya, I'd love to help if you have more questions. Just ask! Wishing you a magical December Disney visit!