Hi Chantal,
What a fantastic time of year to visit, the Christmas decorations are so gorgeous, make sure to visit some of the Disney resorts whilst you're there as they are beautiful too!
On to your question, Mickey's Very Merry Christmas Party is only on select nights so yes you will need to choose a date to go. The MVMCP dates have just been released and in December it will be on on 1, 3, 4, 6, 8, 10, 11, 13 , 15, 17 and 18 at 7pm till midnight. The prices haven't been released yet but last year they were about $55 plus tax for adults 10 and over and $50 for children 3-9. Tickets will be going on sale on May 1st so prices should be released soon.
To book you can call 407-934-7639 and I'm sure that in previous years they have offered online booking via the main
www.disneyworld.com website, I know I did that for the Pirate & Princess party in 2007 so hopefully they'll offer that again to save you an International call.
Have a great time,
Jo