Fortuitous originally meant accidental, but has come to mean lucky. Due to a fortuitous coincidence, you and your best friend's families went on vacation to the same place at the same time.
Fortuitous morphed from "happening by chance" to acquire a second meaning of "lucky chance" probably through its association with fortunate, a similar-sounding adjective. But watch out. If you say fortuitous to mean just plain lucky without the element of chance––that's a usage error.
