Every year the Windows Store, similar to all major retailers, sees a large increase in user downloads and purchases. Last year the holiday months of November, December and January generated 40% more app sales than the average for the year.

To take advantage of this holiday opportunity, you’ll want to update or submit your app no later than November 18 for Thanksgiving (US) and December 15 for the Christmas holiday. Updating your apps now ensures they are ready for other upcoming holidays and festivities around the world.

Microsoft Holiday Submission Deadline for App Publishing by the Holiday
Thanksgiving Thursday,
November 26
November 27
November 18
Christmas Thursday,
December 24
December 25
December 15
New Year’s Friday,
January 1
December 18
I recommend you to submit as early as possible, as volumes are traditionally higher and turn-around times may be a bit longer than what you normally experience as a result.

Source: Window Store app submission deadlines for the upcoming holidays | Building Apps for Windows