The long, slow Sunday lunch that takes half the day. I used to find it inefficient; now it's the anchor of my whole week and I'd fight anyone who tried to take it from me.
There's a local celebration here I'd never heard of before moving, and this year I found myself genuinely excited for it — decorations up, the whole thing. It sneaks up on you: one day a foreign tradition just becomes yours. Which host-country holiday or custom did you not expect to fall for?