California Admission Day is observed in the state of California every September 9 to commemorate the day in 1850 when California joined the United States as the thirty-first state. The holiday marks the anniversary of the passage of An Act for the Admission of the State of California, which was signed into law by President Millard Fillmore. This event followed California's transition from a Spanish colony to a Mexican territory and then to U.S. territory after the Mexican-American War. The influx of immigrants during the California Gold Rush necessitated the establishment of local governance, leading to California's application for statehood. The day is recognized as a legal holiday, celebrating California's admission to the Union and its status as a free state, distinct from National California Day, which is celebrated on February 22.