Christmas can be hard for some people. This is written for The Widows' Handbook, but there are a lot of reasons why people don't want to be forced to be jolly at Christmas.
Don't push people who don't want to be part of the Christmas spirit - people who have been bereaved, particularly if they were bereaved during the Festive season, don't always want to be part of the jollities.
Invite people who struggle with Christmas along to things, and if they say no, accept the answer. Tell them the offer remains open if they change their minds. Accept that they might say yes, and then change their minds. And then give them peace and space.
Remember that if you invite grieving people, you are inviting their grief too - it's part of who they are.
Above everything, be kind.