Christmas lunch is very casual in our family - 16 or so people who all arrive and leave at different times so it would be ok. Half an hour wouldn't matter. If it was a smaller, more formal thing then it would be more important to be there on time. I guess only the OP can judge for their family situation.
DD's routine currently changes daily but I usually can guess that morning when her sleeps will be. If I knew her sleep was going to clash with an important family function I would probably try to avoid it - put her to sleep at the location where the function is, put her down early or go for a huge drive an hour before so she could sleep in the car.
But yes, I have woken her for appointments, etc.