very rarely; once in a blue moon
Emphasizes extreme rarity; stronger than たまに alone. Used in casual and neutral contexts.
He contacts me only very rarely.
I look forward to eating out once in a blue moon.
Mixed kanji-kana spelling; less common but seen in writing.
Full kanji spelling; very rare and may be confused with other readings.
Mixed kana-kanji spelling; uncommon.
Compound of ごく (extremely) and たまに (occasionally). The exact historical derivation is uncertain, but the combination straightforwardly intensifies the rarity.