May was chosen as the observance month to commemorate the arrival of the first Japanese immigrants to the United States on May 10, 1843, and to mark the anniversary of the transcontinental railroad completion on May 10, 1869. The majority of the workers who laid the tracks of that nation-unifying railway were Chinese immigrants.
Department of Defense Education Activity states that in 1978, the AAPI recognition reached the legislative branch of the federal government and Public Law 95-419 was passed by the House and Senate, which designated the week beginning on May 4, 1979 as Asian/Pacific American Heritage Week. In 1979, President Jimmy Carter accepted this call to action and issued Presidential Proclamation 4650 in support of the AAPI week-long observance.
In the years that followed, U.S. Presidents annually issued proclamations for Asian/Pacific American Heritage Week; however, in 1990 President George H.W. Bush extended the week-long acknowledgement to a month-long celebration, Asian/Pacific American Heritage Month. In 1992, President George W. Bush permanently designated the month of May as Asian/Pacific American Heritage Month, via Public Law 102-450. In a 2009 Presidential Proclamation, President Barack Obama expanded the Asian/Pacific American commemoration to include Pacific Islanders and issued his proclamations in honor of Asian American and Pacific Islander Heritage Month.