almost never.

Once in a blue moon I'll have a beer with friends, but it's not my regular drink.

Etymology: based on the informal phrase blue moon (= the second time in one month that you can see the full disk of the moon )

• on the block
• on the blocks
• on the boil
• on the books
• on the brink of
• on the brink of something
• on the bubble
• on the button
• on the case
• on the cheap
• on the contrary
• on the cutting edge
• on the cutting edge of
• on the cutting edge of something
• on the cutting room floor
• on the dot
• on the double
• on the edge
• on the face of it
• on the face of the earth
• on the factory floor
• on the fast track
• on the fence
• on the firing line
• on the fly