Twins: it was my grandfather who died on Christmas Eve. I was five so my remembrance is of my father being gone, then home for Christmas and then gone again for the funeral. That might not have even happened (given the state of air travel in 1965, I can't imagine it did), but that's my memory.
My own father died on Dec. 27/8 (through the night.) When I couldn't reach him by phone (after alerted by his friend who couldn't reach him), I had to s2s before driving over to check on him. In the shower, I remembered my grandfather had died at age 80 at Christmas and knew then my father was dead too (he was almost 87.)
As for holidays being forever overshadowed, dh's father died just before Christmas the following year. He was to come spend Christmas with us (to make up for my Dad's absence) but died before that could happen.
The year after that, I was in a panic and wondering if Christmas would be ruined forever. Trans Siberian Orchestra's heavy metal carols allowed me to at least have some Christmas music playing in the house and the volume and beat reflected my churning emotions. We returned to the Boxing Day party but Christmas Day itself was very subdued.
It has been 15 years this year since Dad died and now the loss/pain at Christmas is no worse than any other day when a thought/memory hits. Every time I step on the Halifax ferry, I tear up (Dad loved the ferry, that's where he bumped into Mom months after meeting her casually. He asked her out on a date before getting off, not wanting to pass up the opportunity a second time.) Sometimes when I pick up our binoculars (one set is from Dad, the other was Dad's.) When I listen to ds describe his new passion for tools and woodworking.
In other words, tearing up will happen whenever, but is no longer worse during certain times.
The key is to get to the point where we can function with the new normal, but still allow our memories to bring us to guilt-free tears. It is a balancing act and time teaches you how to do it.
Nancy and Twins: at some point the anniversary of the deaths take on less significance but I still remember my sister and mom on their birthdays.