Use 'serendipity' when good things happen unexpectedly, not for negative surprises. Getting the last ticket to the concert was serendipity.
Pro Tip 2/3
No Planning Involved
Serendipity involves luck without effort or planning, unlike when you work hard for a result. He met his business partner by serendipity at a café.
Pro Tip 3/3
Unexpected Discovery
'Serendipity' often refers to a discovery or encounter that’s both accidental and beneficial. The discovery of penicillin was a classic case of serendipity.