The phrase "Once in a blue moon" signifies an event that happens with extreme infrequency. Examining the provided choices will clarify its accurate definition:
Frequently: This implies regular or common occurrence, contradicting the idiom's core meaning.
Occasionally: This describes events happening periodically but is less infrequent than the idiom suggests.
Rarely: This aligns precisely with the idiom, denoting an event that happens seldom or not often.
Always: This signifies continuous occurrence, the direct opposite of the idiom's intent.
Following this analysis, the accurate interpretation of "Once in a blue moon" is Rarely.