Bid'a is an act by which a person seeks closeness to Allah, the Blessed and Exalted, in a way that He did not legislate.

This is bid'a. As for customs and worldly matters, these are not called bid'a unless someone seeks closeness to Allah through them and makes them a regular practice.

That is, if he commits to always doing it, this becomes bid'a, to seek closeness to Allah in a way He did not legislate.