There exists among us people who are touched by the Spirits of Christmas at birth. As children, they seem extra kind and extra cheerful (especially around the Holidays). When they turn 16, the Spirits visit the individual and offer them a chance to become a Santa's Helper.
(d20 System rules, presented under the Open Game License. Copyright 2017 by Steve Miller)
During character creation, the player can declare that his character chose to become a Santa's Helper when the Spirits came to visit. (This can also be rolled random if the GM prefers; the character is a Santa's Helper on a roll of 001 on percentile dice.)
In addition to all class abilities and bonus feats, a Santa's Helper gains the following abilities for two days out of the year, December 24 and 25. If Santa requires assistance in delivering gifts to good children, or if there's a particular threat that is stopping him from visiting an area that requires the sort skills and attention that only a player character can bring to bear, the character is asked to spring into action.
Joy of Fellowship: The character and his closest allies (other PCs for example) gain a +1 bonus to all saving throws, attack rolls, and skill checks.
Spellcasting: The character may cast a number of spells from the following list a number of times per day equal to his Wisdom + Charisma bonus. He casts the spell by making a gesture (such as twitching his nose or winking, with no other components required): Animal Friendship, Detect Thoughts, Disguise Self, Fabricate, Knock, Nondetection, Passwall, Telekinesis.
Bonus Feat: The character adds Naughty or Nice to the selection of available bonus feats. The ability gained from this feat is available all year long.
A Santa's Helper is expected to be kind and helpful, and if he is mean or violent toward an undeserving or harmless target, he instantly loses his mystical status.