An Etymological Dictionary of Astronomy and Astrophysics
English-French-Persian

1) A particular time, especially as marked by certain circumstances or occurrences.
2) A special or important time, event, ceremony, celebration, etc.
3) A convenient or favorable time, → opportunity, or juncture (Dictionary.com).

M.E. occasioun from O.Fr. ochaison, ocasion "cause, reason, pretext; opportunity," from L. occasionem (nominative occasio) "appropriate time," from occasum, occasus "fall; sunset," p.p. of occidere "fall down, go down," from ob "down, away" + cadere "to fall," → case.

Âkatan, from prefix â- + katan "to fall," cf. Laki: katen "to fall," kat "he/she fell," beko! "fall!" (an insult); katyâ "fallen;" Lori: kat "event, error;" Kurd. (Soriani): kawtin "to fall, befall," kett "fallen;" Kurd. (Kurmanji): da.ketin "to fall down;" Lârestâni: kata "to fall;" Garkuyeyi: darkat, varkat "he/she fell (sudden death);" Gilaki (Langarud, Tâleš): katan "to fall," bakatam "I fell," dakatan "to fall (in a marsh, in a pit)," vakatan "to fall from tiredness, be exhausted," fakatan "to fall from (i.e., lose) reputation;" Tabari: dakətə "fallen," dakətən "to crash down," dakət.gu "stray cow;" Proto-Iranian *kat- "to fall;" cf. L. cadere "to fall," → case, Pers. kâté.

1) Occurring or appearing at irregular or infrequent intervals; occurring now and then.
2) Intended for supplementary use when needed.
3) Pertaining to, arising out of, or intended for the occasion (Dictionary.com).

→ occasion; → -al.