Keep in mind that when you call in sick on a holiday or any other time where the company is "vulnerable", you just end up screwing one of your fellow pilots. The company WILL find a way to staff the flight and junior manning a senior pilot that has "earned" the right to either have Christmas off or finish early in the day on Christmas is not fair to them, in my opinion.....
FWIW, I am on short call reserve this month, so I am not one of the individuals of whom I speak....