The 6th mission in California was Mission San Francisco De Asi`s, founded by Father Palou on June 9, 1776 . Here's why I think it's the saddest mission in California.

The weather was foggy, sandy and bad for farming. The founder, Father Palou, left the mission to write a book about a friend.
After that Fathers Martin and Aeta came to take the place of Father Palou. Fathers Martin and Aeta were very mean to the Indians. The European missionaries tried to spread Christianity to the Indians by using missions to lure the native people in.

 Then all the Indians got sick and the mission turned into a hospital for the Indians. After the Indians got better, they ran away. The Indians didn't come back for a while so the government thought there was no need for the mission. The government then threatened to shut down and sell the mission. The Indians did not come back because they knew the new Fathers were mean and cruel.

The government put the mission up for sale, but nobody bought it so it remained in the hands of the Mexican government. Fortunately the mission was preserved and is now a national landmark.