Angla-esperanta vortaro »

once in a blue moon signifas en esperanta

AnglaEsperanta
once in a blue moon (very rarely)
adverb
[UK: wʌns ɪn ə bluː muːn]
[US: ˈwəns ɪn ə ˈbluː ˈmuːn]

ege malofteadverb

Via historio