It came in a day game against the Montreal Expos played at Veteran's Stadium. It was the bottom of the 7th inning, with the Phillies trailing 0-1 after future-Phil, then-Expo Tim McCarver drove in Ken Singleton with an RBI groundout. Facing Expos starter Balor Moore (in the midst of a 25-inning scoreless streak) and with Terry Harmon and Roger Freed on base, he launched a three-run jack that put the Phils up 3-1 and ended up winning the game for them. And with that, his first ever home run, on September 16th, 1972--Michael Jack Schmidt had arrived.
Schmidt played in just 13 games after getting called up that September, and he only went deep the one time, ending with just a .206 batting average and three RBIs. His next season didn't go that much better, as he managed just a .196 batting average and struck out 136 times. But credit manager Danny Ozark for sticking with his slugging third baseman, as his 18 homers in his full rookie season doubled to 36 in 1974, and his batting average raised 86 points to .282, meriting Schmidt a sixth-place MVP placing and the first of his 12 All-Star appearances. Before all was said and done, that home run in 1972 marked just the first of Schmidt's 548 total career longballs--then good for seventh-most all-time.
(Thanks again to Broad and Pattison for the assist).