I’m another atheist who celebrates Christmas – but as a family gluttony fest! Sadly my Granddad was a priest, and my parents are hippyligious so there’s a fair bit of that side thrown in, but I can happily ignore that and wait for my chocolate log with cream, ice-cream and clotted cream… Yeah. I know. But it’s just one (three) days a year! We always take big family walks anyway – you can’t eat enough food otherwise! 
I also say Christmas doesn’t start til Christmas eve, but that’s largely cause I’m single and living alone – so no decorations or anything til I visit the parents!