Greetings, Alexandre!
Mickey's Very Merry Christmas Party is a specially ticketed event held on select nights within the Magic Kingdom theme park from 7:00pm-12:00am. In order to take advantage of this unique offering, or watch any of the special events like Mickey's Once Upon a Christmastime Parade, you will need to purchase a Mickey's Very Merry Christmas Party ticket.
If you are wanting to enter Magic Kingdom in the morning during that day's normal operating hours, and attend Mickey's Very Merry Christmas Party, you will need to purchase both a regular admission theme park ticket and a party ticket.
Beginning at 4pm, you will head to one of specially marked locations within Magic Kingdom to have your party ticket (or Magic Band, if you are staying on property) scanned. You will then receive a wristband indicating you are attending the party.
I hope this information is helpful, Alexandre. Please do not hesitate to ask any additional questions you may have as you plan and prepare for a holiday vacation at Walt Disney World® Resort.
Have a magical vacation!
Faith D.