If we’ve been divorced for a while, is it bad to continue celebrating holidays together?

You need to examine your own situation. If it’s not confusing to the kids, as it probably is not, especially after a longer period of doing it, there is probably nothing wrong with continuing. Where it gets a bit difficult is when a new partner has entered the picture and may not want to hang out with the ex or the ex may not want to be around the new partner, especially if the ex doesn’t have a new partner of his or her own yet. However, I still say that holidays are about the kids and preserving their routine, and if the adults are uncomfortable initially, they need to be the mature ones and do the right thing even when they don’t feel like it.