Once a year butter is in the cow's horn.

The saying imagines butter tucked into the cow’s horn during the yearly interval when the cow gives no milk. Without milk, there can be no butter from her.

The horn is a playful place to put the missing butter. The point is a temporary lack of milk and butter, not that butter is a rare treat or merely hard to reach.

Use it as a wry remark about the time when a cow gives no milk and butter is consequently unavailable. It refers to that particular dairy shortage, not to scarcity in general.