How do you unlock Santa on Lego battles

To unlock Santa in LEGO Battles, you must either set your Nintendo DS's date to, or wait for, Christmas (hence why it is unlocked on a "special day").

After you have done this, you must open your LEGO Battles user. Santa should be unlocked.
Santa costs 50 LEGO Studs in the LEGO Store. To earn these, if you haven't already, play a map that you have memorized the location of the LEGO Studs to, or use the Show LEGO Studs Red Brick. Do this 2.5 or 3 times.

After you have done this, you can buy Santa from the LEGO Shop, located in the Hidden Heroes section of the Character Collection.