On November 4, 1841, the first wagon train arrived in California via the Oregon Trail. From the article:via

"Territorial Timeline

The Legend of the Oregon Trail began

In 1841 the first emigrant wagon train headed for California. It left Independence, Missouri on May 1, 1841 and reached California on November 4. Two years later, wagon trains began branching to the north onto the Oregon Trail leading to the Pacific Northwest.

A party of American settlers, led by Dr. Elisha White, arrived safely at the Whitman Mission near present-day Walla Walla in 1842, after having traveled across the country by covered wagon. They brought with them a message for Dr. Marcus Whitman, informing him that his mission was to be terminated. Dr. Whitman rode back east almost non-stop to protest his removal. Having succeeded in stopping his removal, Dr. Whitman traveled home, surveying the trail to Oregon as he in order "to open a practible route and safe passage" for immigrants. Dr. White's success, followed by Marcus Whitman's ride, encouraged prospective Oregon settlers.

In 1843 the largest migration of pioneers to Oregon was underway; by October 1843, the population of the Willamette Valley was increased by 875 new settlers."