Grief and anniversaries, birthdays and Christmas

How do you get through the days that used to be special occasions that you used to love when you're still stuck with grief that's at the forefront of everything. I am dreading Christmas, used to love it and always spent it with family but lost my sibling and now just don't want to face Christmas. It's changed our family so much, my parents are sad all the time and I developed PTSD and no longer join in any gatherings or have people come to my house, can't stand even having a few people in our small home and struggle to go out even in wide open spaces. How do you get through those days