Hi Georgina,
Mickey's Not-So-Scary Halloween Party is an absolute blast and well worth buying an additional event admission ticket for in my opinion.
The dates for 2010 were 14,18, 21, 23, 25, 28 & 30 September, 1, 3, 7, 8, 11, 12, 14, 17, 19, 21, 22, 24, 26, 28, 29 & 31 October and 1 November and tickets usually go on sale at the end of April / beginning of May. I wouldn't however recommend setting any of your plans in stone until the 2011 dates are released.
Each guest is given a trick or treat bag as they enter the Magic Kingdom and you are then able to collect candy from the many trick or treat stations which are set up all around the park.
The trick or treat locations are shown on the halloween park maps which are also available as you enter the park.
Whilst at the party, you will see a special 'Boo-to-You' Halloween Parade which is followed by a spectacular fireworks show called HalloWishes. Lots of Characters and Villains will be dressed in special halloween costumes and many guests will also come in fancy dress. It sure is a lot of fun for the whole family!
Be sure to secure a space to watch the parade in good time and be on the lookout for the Headless Horseman at the start of the parade.
Have a spook-tacular time!