When a second full moon falls in one month, it's literally called a 'blue moon' — rare. Complete the idiom for 'very rarely': 'He calls once in a ___ moon.'

"Once in a blue moon" means very rarely, drawing directly from the real astronomical rarity of a second full moon occurring within a single calendar month, an event uncommon enough to have earned the literal name "blue moon" even though the moon doesn't actually turn blue.

"Red" and "full" don't carry this same rarity association in the fixed idiom — "full moon" happens roughly monthly, the opposite of rare, and "red moon" refers to a different phenomenon (often during a lunar eclipse) unrelated to this particular expression of infrequency.

The astronomical use of "blue moon" itself has an interesting quirk: by one common definition, it isn't the moon's color changing but simply a naming rule for an extra full moon squeezed into an unusually long lunar cycle.