When you become an athiest, do you - like when you became a Christian, have a "day of un-salvation"? Or is it just a mind thing?

I can't speak for anybody except myself. But for me, no. It was a very gradual process, and if i have to choose between "un-salvation" or a "mind thing", it would have to be the latter. Since I no longer believe in salvation in the Christian sense, I have no personal need to"undo" anything.