1. Constitution Day: September 15 is celebrated as Constitution Day in Arizona. On this day, in 1910, the Arizona Constitution was adopted by a vote of the people, making Arizona the 48th state in the Union.

2. Statehood Day: September 15 is also celebrated as Statehood Day in Arizona. This day marks the anniversary of Arizona's admission to the Union as the 48th state.

3. Mexican Independence Day: September 15 is also an important day for Mexicans and Mexican-Americans in Arizona, as it is Mexican Independence Day. On this day in 1810, Father Miguel Hidalgo gave the famous "Grito de Dolores" speech, which sparked the Mexican War of Independence.

4. Arizona's First Indian School: On September 15, 1891, the Fort Mohave Indian Boarding School opened its doors in Bullhead City, Arizona. This was the first Indian school in Arizona, and it served to assimilate Native American children into mainstream American culture.

5. University of Arizona's Founding: On September 15, 1891, the University of Arizona was officially founded in Tucson, Arizona. The university was originally called the Arizona Territorial Normal School, and its mission was to train teachers for Arizona's public schools. Today, the University of Arizona is one of the top public research universities in the United States.