Hi Louise,
It sounds as if you are planning a magical trip during the holidays. This is such an exciting and pixie dusted time of year at Walt Disney World. The decorations are amazing and the sights,sounds, and smells of winter fill the air. Be sure to visit the different resorts on property, if time permits. You won't be disappointed. There is also the Holidays Around the World at World Showcase in Epcot where you can see how these countries celebrate special traditions during this time of year.
In regards to the special party at Magic Kingdom, there is a limited number of tickets for each night of
Mickey's Very Merry Christmas Party. However, there are many dates to choose from as it is offered multiple times a week from early November to just before Christmas. Please be aware that because of its popularity and limited number of tickets, some dates are known to sell out. Since it is July, I see that all the dates are still available at this time. However, I wouldn't wait too long as I can't guarantee it will stay that way. To view the available dates for 2013, click on the link above and go to purchase tickets. This will pull up a calendar of dates for the special event. Keep in mind that tickets are non-refundable and non-transferrable.
Enjoy the spirit of the season on your magical vacation.
~Allison L