Answer by taking a quote from the following article:

Musial was born in Donora, Pennsylvania, the fifth of the six children (four girls and two boys) of Lukasz and Mary (Lancos) Musial (, MOO-show). His mother was of Carpatho-Rusyn descent and his father was a Polish immigrant who chose the name Stanislaw Franciszek for his first son, though his father always referred to Musial using the Polish nickname Stasiu, pronounced "Stashu". Musial frequently played baseball with his brother Ed and other friends during his childhood, and considered Lefty Grove his favorite ballplayer. Musial also had the benefit of learning about baseball from his neighbor Joe Barbao, a former minor league pitcher.

Musial began the 1950s by posting a .350 batting average before participating in the 1950 All-Star Game, where in fan balloting he was the NL's number two choice. He had the longest hitting streak of his career during the 1950 season--a 30-game stretch that ended on July 27. With the Cardinals falling 14 games out of first place by September, manager Dyer used him at first base and all three outfield positions. New Cardinals manager Marty Marion led the team to a third-place finish in 1951, while Musial was named The Sporting News Major League Player of the Year.  National media attention inadvertently turned to Musial a month before the 1952 season began, after Ty Cobb wrote an article regarding modern baseball players that was published in Life magazine. Cobb singled out Musial and Phil Rizzuto as the only players "who can be mentioned in the same breath with the oldtime greats". Cobb went on to refer to Musial as "a better player than Joe DiMaggio was in his prime." In response, Musial displayed his characteristic modesty, saying, "Cobb is baseball's greatest. I don't want to contradict him, but I can't say that I was ever as good as Joe DiMaggio."  The only major league pitching appearance of Musial's career occurred as a publicity stunt during the last Cardinals home game of the 1952 season. Manager Eddie Stanky had a reluctant Musial pitch to Frank Baumholtz, the runner-up to Musial for the best batting average in the NL that season. With Baumholtz batting right-handed for the first time in his career, Musial's first pitch was hit so hard it ricocheted off the shin of third baseman Solly Hemus and into the left field corner. The play was ruled an error, and Musial was embarrassed enough by his complicity in the gimmick to avoid pitching again for the remainder of his career.  The Cardinals franchise was up for sale in early 1953, and Musial and Schoendienst advised their friend and fellow duck-hunter Gussie Busch to consider buying the team. Busch used the resources of the Anheuser-Busch company to purchase the Cardinals, keeping Musial in St. Louis by averting the possibility of a move by the team to another city. The 1953 season marked Musial's 10th NL All-Star selection, and the 12th consecutive time he finished a major league season with a batting average above .300.  Musial accomplished another historical feat on May 2, 1954, in a doubleheader in St. Louis against the New York Giants: he hit three home runs in the first contest, then added two more in the second to become the first major leaguer to hit five home runs in a doubleheader. In addition to his five home runs, he also hit a single in the first game, setting a new record of 21 total bases for a doubleheader. The only player besides Musial to hit five home runs in a doubleheader is Nate Colbert, who achieved the feat in 1972.

what was this feat?
he hit three home runs in the first contest, then added two more in the second to become the first major leaguer to hit five home runs in a doubleheader.