Also known as: うさぎの日, 3月3日はうさぎの日
Rabbit Day is a Date Pun that happens on March 3rd. Its origin comes from goroawase: 3-3 can be read as "mi-mi" which can mean "ear".
It can be called Ears Day for all the animals, but because rabbits have long ears, it gradually became a day dedicated to rabbits.
Don't confuse with Bunny Day which is a date pun related to Playboy Bunny.