Sirius is best man when James Potter marries Lily Evans (PA11).
We don't know exactly when this happened, except that it was sometime between James and Lily leaving Hogwarts (c. 1978 ) and Harry's birth (July 1980). Molly Weasley tells us in HBP5 that couples at this time were getting married very quickly, but we know that James and Lily had an engagement for a time during which they visited Petunia and Vernon Dursley (and quarrelled). We know that Petunia and Vernon were married first (Pm), possibly while Lily and James were still at Hogwarts, so their wedding definitely came later. Working backwards nine months from Harry's birth puts the date some time in 1979.