(Not my original thoughts, but IDK where I found it)
1842Joseph administers the first endowments on the upper floor of his Nauvoo store. The rite consists of washing, anointing, clothing in the garment, and instruction in the signs, tokens, and keywords of the holy priesthood.
1843Eternal marriage (sealings) and the second anointing are instituted. Women receive the endowment for the first time.
1845Under Brigham's leadership, the endowment is performed in the Nauvoo Temple as a ritual drama, with a Creation Room, Garden Room, Telestial Room, Terrestrial Room, and Celestial Room.
1877The first recorded endowments for the dead are performed. Brigham produces the first written text of the endowment, for use in the St. George Temple.
1893Church leaders order minor alterations to the language and procedures of the endowment, trying to ensure greater consistency in how the endowment is administered in different temples.
1904-1906The endowment is publicly scrutinized during Congressional hearings to determine if senator-elect Reed Smoot has taken a treasonable oath. Of particular concern is the "oath of vengeance," added to the ceremony after Joseph Smith's death.
1919-1927A committee appointed by Heber J. Grant produces a revised endowment to be used in all temples. Changes include:
Eliminating the oath of vengeance.
Omitting graphic descriptions from the penalties.
Reducing the number of times the robes of the priesthood are changed from one shoulder to the other.
Discontinuing temple choirs (who had formerly performed the hymn chosen by Lucifer's preacher), in favor of congregational singing.
1923The Church approves a shorter garment for optional use outside the temple (extending to the elbows and knees rather than the wrists and ankles). However, the longer garment remains mandatory for use in the temple.
1936A codified explanation of the symbolism of the marks on the veil is added to the endowment.
1945The endowment is administered in Spanish in the Mesa, Arizona temple, the first time the ceremony is administered in a language other than English.
1950sThe first filmed versions of the endowment are made, for use in the Swiss and New Zealand temples (with different casts for different languages).
1960sFilm becomes the standard medium for presenting the endowment. Filmed endowments take on a theatrical quality (with costumes, scenery, music, etc.) and are dubbed from English into other languages.
As the filmed endowment makes congregational singing awkward, the preacher's hymn is discontinued.
1970s Revisions are made to the portion of the ceremony involving Lucifer's preacher: Lucifer no longer specifies the amount of the preacher's salary, and a reference to Satan's having black skin is omitted.
1975 The long, pre-1923 garment becomes optional in the temple and is eventually discontinued.
1978 The lifting of the priesthood ban on blacks makes the endowment available to all Latter-day Saints, regardless of race.
April 1990 Major revisions are made:
All penalties, the five points of fellowship, and syllables purported to having meaning in the Adamic language are omitted.
The part of the preacher is eliminated, as well as a reference to Lucifer's "popes and priests."
Women no longer covenant to obey the law of their husbands.
Language which faults Eve for initiating the Fall is dropped.
Many references to Adam are replaced with references to Adam and Eve.
The lecture at the veil is discontinued.
Orders from Elohim are repeated fewer times for brevity's sake.
2005 Procedures for the initiatory are revised such that initiates clothes themselves in the garment before entering the washing room, thus eliminating the final vestiges of ritual nudity (which had been curtailed by introduction of the shield, probably during the 1920s). Water and oil are applied to the head only, not to multiple parts of the body.
2008 Initiates are no longer instructed to stand while making covenants.