One of my client's (let's call her Libby) was stressing. Divorced for less than a year, she hadn't yet had to negotiate a holiday season and holiday events. She and her ex had mutually agreed to their divorce but she was still dealing and healing. They only talked when handing off the children and then only about the children's needs.
"What do I do if he shows up at the same parties?" she asked nervously. The question isn't "if." It's "when." They have extended families who love them both and who have wisely not taken sides. They have the kind of friends (also wise) who invite them both and let them work it out. They will inevitably run into each other at their children's holiday concerts and school programs and at some adult holiday parties.
A useful therapists' technique is "reframing." Just as you can change the look of a picture by changing the frame (Bigger? Smaller? Plainer? Move it over a bit?), you can change how you "see" a situation by framing it differently. Libby can frame the distinct possibility that she'll run into her ex as "OMG. He's there! What will I do?" or she can frame it as, "Ho-hum, there's going to be another person I'd rather not talk to at my aunt's eggnog party."
The truth is, we all have friends who have friends we don't particularly like. They regularly show up at our friend's events and celebrations. Our friend finds them fascinating but they simply aren't the type of folks we would seek out, much less get close to. Still, they are our friend's friends so we're polite when we run into them. We make gracious small talk for a few minutes. Then we find an excuse to go find someone more compatible to talk to.
Libby already knows how to do it. She's done it countless times; not only at her good friend's home but at the office, at the kids' school open house, and in the grocery aisle. She's run into someone she knows but doesn't particularly like, has made polite excuses, and exited the situation.
Framed this way, her ex is simply someone else she would rather not talk to who is going to show up now and then in her life. Her best holiday strategy is to say a polite "hello, nice to see you" and to make an equally polite exit to talk to a friend or to get another glass of eggnog.