INFREQUENT, unfrequent [rare], seldseen [archaic], uncommon, sporadic; rare, rare as a blue diamond; few [See Fewness]; scarce; almost unheard of, scarce as hens teeth [colloq.], unprecedented, which has not occurred within the memory of the oldest inhabitant, not within ones previous experience.
SELDOM, rarely, scarcely, hardly; not often, unfrequently, infrequently, uncommonly, sparsely, unoften; scarcely ever, hardly ever; once in a blue moon [colloq.].
once; once for all, once in a way; pro hac vice [L.].