You had to do housework around Santa's house. Muck out the reindeer stables, stuff the turkey, light the fire, chase out the burglars sort-of-thing. Upon completion of said chore you would expose a reward like 10 free-spins or a 50% match offer etc. As said above you could dodge the housework and press a 'reveal' button to show it anyway. It was actually quite a clever and entertaining addition to the site IMO. Most of the 24 days' offers were dependent on making a deposit of £20+ and you played all your accumulated free spins Christmas Day.
As the OP said there was apparently a 'surprise' incentive for completing all 24 days' deposits/housework which has yet to be described by anyone. Maybe they're still working it out based on returns from the calendar, I don't know but it's GUTS and they won't let you down - OP will advise us when he knows the answer presumably.