1. Statehood Day: September 21, 1959, marks the day when Hawaii became a state of the United States of America. It is celebrated as Statehood Day in Hawaii, where many festivities take place across the state.

2. Battle of Moku'ōhai: September 21, 1782, was the date of the Battle of Moku'ōhai, which was fought between Kamehameha I and Keoua Kuahuʻula on the Big Island of Hawaii. It is considered to be one of the most significant battles in Hawaiian history.

3. Makahiki Season: In ancient Hawaii, September 21 marked the start of the Makahiki season. This was a time of peace and harvest where people celebrated the Hawaiian god Lono and engaged in various sports and games.

4. Hurricane Iniki: September 21, 1992, was the day when Hurricane Iniki struck the Hawaiian islands, causing widespread destruction and loss of life. It was one of the most powerful and destructive hurricanes to ever hit Hawaii.

5. Aliʻi nui Kualiʻi: September 21, 1736, is the birth date of Aliʻi nui Kualiʻi, who was the last king of Kauai before it was forcibly annexed by Kamehameha I. Kualiʻi was known for his resistance to Kamehamehaʻs attempts to unify the Hawaiian islands under his rule.