How utterly cruddy of them. How disrespectful, hurtful, and unthoughtful.

I've a few ideas. Get in there and make the invitations as early as possible. Put them on the spot; if they say they already have plans then tell them when the next holiday will be planned and that you expect them to attend. Put the ball in their court and ask them when they are available to celebrate the holiday in question. What a great opportunity to establish new traditions. Celebrate alternative days. for instance instead of Christmas have a Solstice dinner or Boxing Day luncheon.