Use 'serendipitous' for unexpected events that turn out to be advantageous or bring joy. Finding the rare book in the street market was a serendipitous stroke of luck.
Pro Tip 2/3
Not Always Big
'Serendipitous' applies to both minor and significant beneficial events, not just life-changing ones. It was serendipitous to discover my favorite song playing on the radio.
Pro Tip 3/3
Accidental Discovery
Often used in scientific or creative contexts to describe the discovery of something valuable by chance. The serendipitous discovery of penicillin changed the course of medicine.