On June 30, 1978 On an 0-2 pitch from Atlanta Braves pitcher Jamie Easterly, Giant first baseman Willie McCovey launches his 500th career home run over the left field fence at Fulton County Stadium becoming the 12th player to accomplish the feat. The historic homer occurs in the 2nd inning of the opener of a doubleheader which the Giants sweep, 10 – 9 and 10 – 5. Mike Ivie adds his second pinch grand slam of the year in the opener. Jack Clarkhas three home runs in the two games.

Source
Baseball Reference June 30