Mom and Dad need to figure out a plan then tell the kids and the extended family. If you need help figuring out a plan on how to share the various holidays then see a mediator. A mediator can help you resolve all the issues of a divorce or just the ones that you want to address. After you have a plan, be very clear with your support system of friends and family that you need kindness and compassion, not guilt and anger, from them. Yes things will be different but they do not have to be bad.
As I am writing this I keep thinking of the song Tradition from Fiddler on The Roof. Even Tevya saw that while old traditions provide comfort, sometimes it is necessary to forge a new path and create new traditions for everyone to enjoy.