"In like a lion, out like a lamb."
Turns out it has been around forever! The Paris Review claims that one of the earliest know use of the phrase is from 1732. Here are some theories of where it may come from:
Changing of Seasons
When March starts, it’s still winter, and by the end of the month spring has started. Though technically true, the Spring Equinox (for our hemisphere) is March 20 this year. However, it usually doesn't feel like spring until well into April.
The Balance of Life and Weather
The long held belief that things should balance. So, if a month started with bad weather, it should end with nice, calm weather.
Jesus's first appearance was as the sacrificial lamb, but he then returns as the Lion of Judah. Like in Narnia! I hadn't heard of the Lion of Judah before, but this theory puts the animals in the wrong order.
Horoscopes and Constellations
At the start of March, Leo, or the lion sign, is the rising star constellation and by April, it’s Aries. Aries is a ram, a male sheep. (And lambs are baby sheep!) This one is my favourite theory.