From the official page:
Asian Americans and Pacific Islanders have a rich heritage thousands of years old and have both shaped the history of the United States and had their lives dramatically influenced by moments in its history. Every May during Asian American and Pacific Islander Heritage Month and throughout the year, the National Park Service and its partners share those histories and the continuing culture thriving in parks and communities today.
[Asian Pacific American Heritage Month is] a celebration of Asians and Pacific Islanders in the United States. A rather broad term, Asian/Pacific encompasses all of the Asian continent 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).
You can find more information at several governmental agencies official pages for the Heritage Month: