It'll be a long day in January
Use it to describe something that will never happen. When hell freezes over has the same meaning.
It'll be a long day in January when you beat me at tennis.
Mad as a March hare
Acting crazy. Hares, which look like rabbits, just around and act crazy during their March breeding seson.
I like Jimmy but he acts as mad as March hare sometimes.
April showers bring May flowers
Although rain in April is annoying, it starts the flowers growing.
We all hate this rainy weather but remember April showers bring May flowers.
May-December romance
When one person in a relationship is a lot older than the other, it is a May-December romance.
People are always skeptical of May-December
Maybees don't fly in June
Use it to when telling someone to stop changing their mind so much. "Maybees" is a pun on "maybes.
I wish you would make decision and stick to it. Maybees don't fly in June!
A cold day in July
Use it to describe a situation or event that will never happen. It has the same usage as It'll be a long day in January (above).
It'll be a cold day in July when that happens.