According to asianpacificheritage.gov, "The month of May was chosen to commemorate the immigration of the first Japanese to the United States on May 7, 1843, and to mark the anniversary of the completion of the transcontinental railroad on May 10, 1869. The majority of the workers who laid the tracks were Chinese immigrants."
In 1978, Congress passed Public Law 95-419, which directed President Carter to designate a week in May 1979 as Asian/Pacific American Heritage Week. In 1990, Congress passed a law amending the 1978 law, requesting a month-long observance.
The term "Asian/Pacific" includes the continent of Asia and the Pacific islands of Melanesia (New Guinea, New Caledonia, Vanuatu, Fiji and the Solomon Islands), Micronesia (Marianas, Guam, Wake Island, Palau, Marshall Islands, Kiribati, Nauru and the Federated States of Micronesia) and Polynesia (New Zealand, Hawaiian Islands, Rotuma, Midway Islands, Samoa, American Samoa, Tonga, Tuvalu, Cook Islands, French Polynesia and Easter Island).