"The Star-Spangled Banner" was recognized for official use by the United States Navy in 1889, and by U.S. President Woodrow Wilson in 1916, and was made the national anthem by a congressional resolution on March 3, 1931 (46 Stat. 1508, codified at 36 U.S.C. § 301), which was signed by President Herbert Hoover.

When did the star spangled banner became a national anthem? OK. Let's think. My stream of consciousness:
The relevant sentence in the passage is: "The Star-Spangled Banner" was recognized for official use by the United States Navy in 1889, and by U.S. President Woodrow Wilson in 1916, and was made the national anthem by a congressional resolution on March 3, 1931 (46 Stat. 1508, codified at 36 U.S.C. § 301), which was signed by President Herbert Hoover. The answer is March 3 , 1931.