May was first designated as Asian-Pacific American Heritage Week in 1978 when Congress passed Public Law 95-419, initiating the annual celebration. It is a month to commemorate generations of Asian and Pacific Islanders who have contributed to build a dynamic and diversified American society and culture.
It is also not without reason the month of May was chosen. The transcontinental railroad was completed in May, 1869, paving the way for Western expansion, generating fortunes for influential men and representing an epic achievement in American history. The majority of the railroad workers were Chinese immigrant laborers who toiled in severe weather and brutal working conditions to lay the tracks. Also, the first Japanese immigrants came to the United States in May, 1843. These are the two historic facts mentioned in Public Law 102-450, which was passed by Congress in 1992 to permanently designate May of each year as “Asian-Pacific American Heritage Month.”
If you are interested in reading further, the Eisenhower Library has a rich collection of books and electronic resources on Asian Americans in the United States.