1. Mexican Independence Day: September 16 is celebrated as Mexican Independence Day in California, marking the day in 1810 when Mexico began its fight for independence from Spain. Many Californians of Mexican descent celebrate the day with parades, festivals, and traditional dances.

2. California Constitution: On September 16, 1849, California's first constitution was signed in Monterey. The constitution paved the way for California to become a state in the union the following year.

3. San Francisco Earthquake: On September 16, 1957, a 5.7 magnitude earthquake struck the San Francisco Bay Area. The quake caused damage to buildings and infrastructure but resulted in no fatalities.

4. Silicon Valley Innovations: Many of the world's most innovative companies were founded in Silicon Valley, and September 16 marks several important milestones for these companies. On this day in 1998, Google was incorporated in California. In 2002, eBay announced that it was acquiring PayPal, which would go on to become one of the world's most popular online payment systems.

5. California State Parks Day: In honor of California's state parks, which cover over 1.6 million acres of land and host millions of visitors each year, September 16 is celebrated as California State Parks Day. On this day, admission to all state parks is free, and many parks host special events and activities.