He stopped on a picture of his parents' wedding day. There was his father waving up at him, beaming, the untidy black hair Harry had inherited standing up in all directions. There was his mother, alight with happiness, arm in arm with his dad. And there ... that must be him. Their best man... Harry had never given him a thought before ... His face wasn't sunken and waxy, but handsome, full of laughter. (PA11)
Sirius Black is their best man (PA11). Lily’s sister Petunia refused to attend the wedding because her husband Vernon did not get along with James Potter (Pm).
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.