advent (n.)

"one of a religious denomination that believes in or looks for the early second coming of Christ to establish a personal reign," 1843; see advent + -ist. In Church Latin adventus was applied to the coming of the Savior, both the first or the anticipated second, hence Adventist was applied to millenarian sects, especially and originally the Millerites (U.S.). By the end of the 19c. there were three main divisions of them; the Seventh-Day Adventists (by 1860, see seventh) were so called for their observation of Saturday as the Sabbath.

"of the nature of an addition from without, not from the essence of the subject; accidentally or casually acquired," c. 1600, from Medieval Latin adventitius "coming from abroad, extraneous," a corruption of Latin adventicius "foreign, strange, accidental," from advent- past-participle stem of advenire "to arrive at, reach, come to" (see advent). Related: Adventitiously; adventitiousness.