Very occasionally (4,2,1,4,4)
I believe the answer is:
once in a blue moon
'very occasionally' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for once in a blue moon that I've seen before include "very rarely" , "with extremely low frequency'" , "Hardly ever (4,2,1,4,4)" , "on very rare occasions" , "very occasionally" .)