1848 – California Gold Rush: James W. Marshall finds gold at Sutter's Mill near Sacramento.
The California Gold Rush (1848–1855) began on January 24, 1848, when gold was discovered by James Wilson Marshall at Sutter's Mill, in Coloma, California. News of the discovery soon spread, resulting in some 300,000 men, women, and children coming to California from the rest of the United States and abroad. Of the 300,000, approximately 150,000 arrived by sea while the remaining 150,000 arrived by land.
Image & text:wikipedia.com
Note: the date is in the first month of 1848. But we don't say the "48'ers." That's because the people who discovered gold tried to keeep quiet about it because they figured they'd be overrun (and they were.) But more importantly, communication and transportation was so slow in the middle of the 19th Century that it took almost a full year for momentum to build for fortune seekers from around the world to come to California. But come they did. The population of the newly renamed town of San Francisco grew from 800 to 80,000 in a single year!