Norm, you said: "Do I really want to do this to her right before Christmas?...Maybe I should let her enjoy her Christmas and approach her after?"
I know this is a horrible decision and a horrible thing you are going through, but can I just throw out there the fact that if you DON'T expose them BEFORE Christmas, then her husband is going to be sneaking off to screw your wife sometime on or around Christmas. They are affair partners, and they will likely plan to spend some special time together for the holiday (affair partners always feel that they deserve to get holiday jollies from each other). And not only that, he may screw your wife then go home and screw his own wife on Christmas.
So which would be worse? She spends her Christmas knowing the awful truth, but at least the activity has stopped....or she spends her Christmas knowing nothing while the two A partners are crapping all over her life on Christmas by screwing each other while she's hanging ornaments with their children on the tree?
I know it sucks, but the less painful option is knowing about it but at least also knowing you've stopped it from happening (for the time being).
