I have not received instructions one way or the other so I default to Happy Holidays or Merry Christmas, depending on the circumstance.
Considering the slew of holidays, religious and otherwise, that occur this time of the year, is it really a big of a deal? Happy Holidays makes more sense anyway since typically people don't wish someone a pleasant particular holiday except on the actual day of the holiday.
A human being should be able to change a diaper, plan an invasion, butcher a hog, conn a ship, design a building, write a sonnet, balance accounts, build a wall, set a bone, comfort the dying, take orders, give orders, cooperate, act alone, solve equations, analyze a new problem, pitch manure, program a computer, cook a tasty meal, fight efficiently, and die gallantly. Specialization is for insects. - Robert A. Heinlein