It was still unknown outside the Smalltalk community, so it's fair to give JUnit the credit for spreading the idea more. JUnit was born on a flight from Zurich to the 1997 OOPSLA in Atlanta. Kent was flying with Erich Gamma, and what else were two geeks to do on a long flight but program?
OOPSLA was October 5-9, 1997. So it's a good bet this flight was in the first few days of October. That's close enough to an actual date to observe it.
Isn't the Internet wonderful? I never would have found this out at a library!