1. The Summer Solstice: June 21 marks the Summer Solstice, the longest day of the year in terms of daylight hours in the Northern Hemisphere. In California, this day is generally marked by events such as solstice festivals, beach parties, and bonfires at popular locations such as Santa Monica Beach and Huntington Beach.

2. Earthquake in 1915: On June 21, 1915, a powerful earthquake struck the Owens Valley in California, causing significant damage to several towns and communities. The earthquake measured 6.1 on the Richter scale, and was felt across a wide area of Eastern California.

3. International Surfing Day: June 21 is recognized as International Surfing Day worldwide. Californians celebrate this day by taking part in various events and activities such as surf contests, community cleanups, and beach parties.

4. Legalization of Same-Sex Marriage: On June 21, 2008, same-sex marriage became legal in California after a court ruling overturned a ban on gay marriage. Thousands of couples tied the knot in the months following the ruling, until Proposition 8 was passed later that year, which again banned same-sex marriage.

5. Founding of the California Missions: June 21, 1769 was a momentous day in California history, as it marked the founding of the first of 21 Spanish missions that would be established in California. Father Junipero Serra established the Mission San Diego de Alcala, which served as the headquarters for the Spanish missionary effort in California. The missions played a vital role in the economic, cultural, and religious development of California over the centuries.