1. Arizona became the 48th state in the United States on February 14, 1912, but it was on May 15th of that same year that Governor George W.P. Hunt officially signed the state Constitution, thereby completing the transition from a territory to a state.

2. On May 15, 1930, the Hoover Dam began construction on the Colorado River on the border between Arizona and Nevada. The dam, which was completed in 1936, not only revolutionized hydroelectric power but also provided water to an area covering over 1 million acres in both Arizona and California.

3. In 1940, the Arizona State Teachers College (now known as Northern Arizona University) awarded its first master's degree to a student named Mary G. Boyer on May 15th. This marked a significant milestone in the university's history, as it had only been established as a teacher's college twenty-nine years earlier in 1911.

4. May 15, 1955, was a day of celebration in Arizona as the Arizona Cardinals football team, at the time known as the Chicago Cardinals, held a groundbreaking ceremony for a new stadium in Phoenix. The stadium, which was completed in 1958 and ultimately named Sun Devil Stadium, became the home of the Arizona State Sun Devils football team and served as an important venue for sporting events and concerts in the greater Phoenix area.

5. On May 15, 2002, the Phoenix Suns basketball team played their final game at the historic Veterans Memorial Coliseum before moving into the new Arizona Veterans Memorial Arena. The Coliseum had been the home of the Suns since 1968 and was also the site of numerous concerts, rodeos, and other events over the years. The venue remains an important part of Arizona's cultural history to this day.