This poem is from the proverbial distichs of Georgius Carolides (1569-1612), which you can read online at the University of Mannheim.
Occasio Quaerenda
Si non prompta venit, quaerenda occasio facti est,
Quae, dimissa semel, non reditura, fugit.
The vocabulary is keyed to the DCC Latin Vocabulary list. There are two words in this poem that are not on the DCC list:
ocāasio (occāsiōnis, f.): opportunity, chance
promptus, -a, -um: visible, in readiness
dīmittō -mittere -mīsī -missum: send away
faciō facere fēcī factum: do, make
fugiō fugere fūgī fugitum: flee, escape
nōn: not
quaerō -rere -sīvī-situm: seek, inquire
qui quae quod: who, which, what / quis quid: who? what? which?
redeō -īre -iī -itum: go back, return
semel: once
sī: if
veniō venīre vēnī ventum: come