I would not enter into a discussion about whether you should go snowboarding--you do what you think is best during your custody period." Of course, consider if it's safe and legal to go, and use the tools available to you to keep your kids as safe as possible (e.g., social distancing, masks, testing).
If I understand correctly, she negotiated a specific week of custody, and now she wants to change it. Note her wording, skipping the trip would "lose her time". If this was a fair deal where she got X time and you got Y time, I'd probably work with her, "Yes, we only agreed to this date. What alternate date(s) did you have in mind?" If this was an unfair deal where you gifted her time, I wouldn't be so generous. "Yes, we only agreed to this date. I am open to an exchange of weeks in the future when the timing works for both of us."
Spending Christmas together sounds awkward. Not too late to abort? If you go ahead, treat it like any negotiation. Say what you want. Listen to what she wants. Find middle ground or take turns getting your way.
May'19 - separation. | Dec'19, Oct'20, Jan'21 - painful breakups. | Jan'21 - freedom! "We the ones who play hard, we live hard, we love hard, we light up the dark." -- Kesha