In 1978 a joint congressional resolution established Asian/Pacific American Heritage Week. The first 10 days of May were chosen to coincide with two important milestones in Asian/Pacific American history: the arrival in the United States of the first Japanese immigrants (May 7, 1843) and contributions of Chinese workers to the building of the transcontinental railroad, completed May 10, 1869.
In 1992 Congress expanded the observance to a monthlong celebration. Per a 1997 Office of Management and Budget directive, the Asian or Pacific Islander racial category was separated into two categories: one being Asian and the other Native Hawaiian and Other Pacific Islander.
Check out the US Census Facts for Features page on Asian Pacific Americans.