Juan Crespí, as a member of the 1769 Spanish Portolà expedition, authored the first written account of interaction between Europeans and the indigenous population in the region that today makes up Orange County.
The expedition arrived at the site from the northeast, traveling down San Juan Creek, and camped near the future mission site on July 23.
At the time, Crespi named the campsite after Santa Maria Magdalena (though it would also come to be called the Arroyo de la Quema and Cañada del Incendio, "Wildfire Hollow").

In early 1775, Don Antonio María de Bucareli y Ursúa, Viceroy of New Spain, authorized the establishment of a mission at a logical halfway point between Mission San Diego de Alcalá and Mission San Gabriel Arcángel.
By that time, the site was already known by the name of its patron saint, "San Juan Capistrano".

Up from the south slow filed a train,

Priests and Soldiers of Old Spain,

Who, through sunlit lomas wound

With cross and lance, intent to found

A mission in the wild to John

Soldier-Saint of Capistrano.

— Saunders and Chase, The California Padres and Their Missions, p. 65
At the proposed site, located approximately 26 leguas (Spanish Leagues) north of San Diego, 18 leagues south of San Gabriel, and half a league from the Pacific Ocean, an enramada (arbor) was constructed, two bronze bells were hung from the branch of a nearby tree, and a wooden cross was erected.
The grounds were consecrated by Fermín Lasuén of Mission San Carlos Borromeo de Carmelo on October 30, 1775 (the last day of the octave after the feast of San Juan Capistrano), near an Indian settlement named "Sajavit"; thus, La Misión de San Juan Capistrano de Sajavit was founded.

Assisting clergy Gregório Amúrrio of Mission San Luis Obispo arrived from San Gabriel eight days later with a supply of goods and cattle.
Unfortunately, word arrived from San Diego at the same time that a group of natives attacked the mission and brutally murdered one of the missionaries (Luís Jayme).
Since it was feared at the time that any hostile action by the natives against the few burgeoning outposts might break Spain's tenuous hold on Alta California, the priests quickly buried the San Juan Capistrano Mission bells.
Lieutenant José Francisco Ortega, military leader of the expedition, led all but a small contingent of Spanish soldiers back to El Presidio de San Diego to help quell the uprising; the priests, along with the few remaining soldiers as an escort, gathered up their belongings and fled to the safety of the Presido, where they were given further details of the disaster.

 
One year later Serra himself, along with Amúrrio and Pablo de Mugártegui, took up work on the Mission at San Juan Capistrano; the contingent, accompanied by eleven soldiers, arrived on October 30 or 31, 1776.
Upon their return to the site today known as "Mission Vieja," the party excavated the bells and constructed a new arbor; the original wooden cross was, to their surprise, still standing.
Serra celebrated High Mass in thanksgiving on November 1, 1776—celebrated ever since as the official founding date.
Due to an inadequate water supply the Mission site was subsequently relocated approximately three miles to the west near the Indian village of Acágcheme.
The new venue was strategically placed above two nearby streams, the Trabuco and the San Juan.
Mission San Gabriel provided cattle and neophyte labor to assist in the development of new the Mission.
Amúrrio performed the Mission's first baptism on December 19 of that year (a total of 4,639 souls were converted at the Mission between 1776 and 1847.)
The first Indian marriage was blessed by Mugártegui on the feast of the "Espousals of the Blessed Virgin Mary," January 23, 1777.
Mugártegui also presided over the first burial ceremony on July 13 (the first burial on Mission grounds would not take place until March 9, 1781).
The Registers of Baptisms, Marriages, and Burials are all intact and preserved at the Mission, as is the Confirmation Register (San Juan Capistrano is one of the few Missions to have retained this document).
Juan Crespí, as a member of the 1769 Spanish Portolà expedition, authored the first written account of interaction between Europeans and the indigenous population in the region that today makes up Orange County. The expedition arrived at the site from the northeast, traveling down San Juan Creek, and camped near the future mission site on July 23. At the time, Crespi named the campsite after Santa Maria Magdalena (though it would also come to be called the Arroyo de la Quema and Cañada del Incendio, "Wildfire Hollow").
In early 1775, Don Antonio María de Bucareli y Ursúa, Viceroy of New Spain, authorized the establishment of a mission at a logical halfway point between Mission San Diego de Alcalá and Mission San Gabriel Arcángel. By that time, the site was already known by the name of its patron saint, "San Juan Capistrano".
Up from the south slow filed a train,

Priests and Soldiers of Old Spain,

Who, through sunlit lomas wound

With cross and lance, intent to found

A mission in the wild to John

Soldier-Saint of Capistrano.
— Saunders and Chase, The California Padres and Their Missions, p. 65
At the proposed site, located approximately 26 leguas (Spanish Leagues) north of San Diego, 18 leagues south of San Gabriel, and half a league from the Pacific Ocean, an enramada (arbor) was constructed, two bronze bells were hung from the branch of a nearby tree, and a wooden cross was erected. The grounds were consecrated by Fermín Lasuén of Mission San Carlos Borromeo de Carmelo on October 30, 1775 (the last day of the octave after the feast of San Juan Capistrano), near an Indian settlement named "Sajavit"; thus, La Misión de San Juan Capistrano de Sajavit was founded.
Assisting clergy Gregório Amúrrio of Mission San Luis Obispo arrived from San Gabriel eight days later with a supply of goods and cattle. Unfortunately, word arrived from San Diego at the same time that a group of natives attacked the mission and brutally murdered one of the missionaries (Luís Jayme). Since it was feared at the time that any hostile action by the natives against the few burgeoning outposts might break Spain's tenuous hold on Alta California, the priests quickly buried the San Juan Capistrano Mission bells. Lieutenant José Francisco Ortega, military leader of the expedition, led all but a small contingent of Spanish soldiers back to El Presidio de San Diego to help quell the uprising; the priests, along with the few remaining soldiers as an escort, gathered up their belongings and fled to the safety of the Presido, where they were given further details of the disaster.
 
One year later Serra himself, along with Amúrrio and Pablo de Mugártegui, took up work on the Mission at San Juan Capistrano; the contingent, accompanied by eleven soldiers, arrived on October 30 or 31, 1776. Upon their return to the site today known as "Mission Vieja," the party excavated the bells and constructed a new arbor; the original wooden cross was, to their surprise, still standing. Serra celebrated High Mass in thanksgiving on November 1, 1776—celebrated ever since as the official founding date. Due to an inadequate water supply the Mission site was subsequently relocated approximately three miles to the west near the Indian village of Acágcheme. The new venue was strategically placed above two nearby streams, the Trabuco and the San Juan. Mission San Gabriel provided cattle and neophyte labor to assist in the development of new the Mission. Amúrrio performed the Mission's first baptism on December 19 of that year (a total of 4,639 souls were converted at the Mission between 1776 and 1847.) The first Indian marriage was blessed by Mugártegui on the feast of the "Espousals of the Blessed Virgin Mary," January 23, 1777. Mugártegui also presided over the first burial ceremony on July 13 (the first burial on Mission grounds would not take place until March 9, 1781). The Registers of Baptisms, Marriages, and Burials are all intact and preserved at the Mission, as is the Confirmation Register (San Juan Capistrano is one of the few Missions to have retained this document). 
Mission San Juan Capistrano