occasionyoudaoicibaDictYouDict[occasion 词源字典]
occasion: [14] Like English befall, occasion depends on a metaphorical connection between ‘falling’ and ‘happening’. Its ultimate source is the Latin verb occidere ‘go down’, a compound formed from the prefix ob- ‘down’ and cadere ‘fall’ (source of English cadence, case ‘circumstance’, decadent, etc). The figurative notion of a ‘falling together of favourable circumstances’ led to the coining of a derived noun occasiō, meaning ‘appropriate time, opportunity’, and hence ‘reason’ and ‘cause’.

English acquired it via Old French occasion. Also from Latin occidere comes English occident [14], a reference to the ‘west’ as the quarter in which the sun ‘goes down’ or sets.
=> cadaver, cadence, case, decadent, occident[occasion etymology, occasion origin, 英语词源]
occasion (n.)youdaoicibaDictYouDict
late 14c., "opportunity; grounds for action, state of affairs that makes something else possible; a happening, occurrence," from Old French ochaison, ocasion "cause, reason, excuse, pretext; opportunity" (13c.) or directly from Latin occasionem (nominative occasio) "opportunity, appropriate time," in Late Latin "cause," from occasum, occasus, past participle of occidere "fall down, go down," from ob "down, away" (see ob-) + cadere "to fall" (see case (n.1)). The notion is of a "falling together," or juncture, of circumstances.
occasion (v.)youdaoicibaDictYouDict
mid-15c., "to bring (something) about," from occasion (n.), or else from Old French occasionner "to cause," from Medieval Latin occasionare, from Latin occasionem (see occasion (n.)). Related: Occasioned; occasioning.