Juan Soto of the Washington Nationals hit his first major league home run five days before he played his first major league game.
On June 18, 2018, he hit a two-run homer that was logged as occurring on May 15. That’s because the June 18 game was a continuation of a May 15 game that had been suspended due to rain. When such a game is resumed, all the statistics are recorded with the date of the original game.
So Soto is credited with his first major league hit and his first major league home run on May 15, five days before he made his major league debut.