First off, this does not mean that the Celtics will play on Christmas Day in 2015--there are uniform mockups for every team.
Here's a little more about them from UniWatch.com (which has detailed info and pictures of others):
The script chest insignia are meant to evoke the feel of the fancy script on a Christmas card.
It’s hard to see on the mock-ups, but the NBA logo above the NOB will be shown in a splotch of sealing wax, which is meant to evoke the feel of an old-fashioned Christmas card.
There will supposedly be “glazed sugar ornamental shiny surfaces” on the fabric and trim.