Christmas: I cook, therefore I get to say what we're eating! I do sometimes go off plan and eat carby stuff. But we don't have turkey and all the trimmings, so it's easier. We generally have something like a whole fillet of beef, with different veg. Sometimes I might have a roast potato or two, or possibly a Yorkshire pudding.
But I try to stay low carb as much as I can! I'm not a great pudding eater, so that's not an issue. My only real temptation is if someone has bought me nice chocolates - by which I don't mean Milk Tray or Roses, but stuff like Hotel Chocolat. The alcohol is the biggest downfall.
Birthdays (or other celebrations) generally the same - I've been low carbing for such a long time that it's pretty routine, to be honest. If I eat a dessert, it's because I really want one, and I don't beat myself up about it.
Social occasions are much easier, because I actively seek out low carb options. I have no qualms about asking restaurants to substitute things - salad for chips, for example. And I've never had a restaurant refuse to do that.
Going to other people's houses is a different matter. Generally - if they're really good friends - they know I low carb, or I tell them. But often they don't understand what that means! In which case I just go with the flow and eat what they have provided.
If you're going to adopt this WOE for the long term, you do have to accept that there will be certain situations where it's not possible - but not stress about it, and get back 'on the wagon' as soon as you can.