It is March 7th - and seven is generally considered a lucky number. Lucky Seven goes back to ancient Greece and the mathematician Pythagoras, who decreed the number seven lucky, which would cement in Western culture a belief that also reigns in Asian and the Middle East. Pythagoreans called the number 7 "the Septad".
Seven is all around us: there are Seven Seas, Seven Heavens, Seven Continents, Seven Colours in a Rainbow, Seven Notes on a musical scale, Seven Days in a week, Seven Wonders of the World and so on.
Are you old enough to remember celebrating 07/07/77? And more recently there's 07/07/07. The website TheKnot.com had 38,000 couples register 07/07/07 as their wedding date.
A seminal paper was published in 1956 by the psychologist George A Miller called “The Magical Number Seven, Plus or Minus Two”. Miller claims that it is more than just coincidence that the number 7 seems to be all around us.
Our immediate memory has been shown to perform well when remembering up to, and no more than, seven things. We can distinguish and make a judgement about seven different categories, and remember around seven objects at a glance.
Looking up Lucky in Wikipedia, I found a reference to Ronald Regan's dog, Lucky. Clicking the link, I came to the listing of pets of the presidents.
I was impressed with the range of Calvin Coolidge's pets. The list is about 30 pets long. Here are a few: Goldy - a "yellow bird" Enoch - Goose Smoky - Bobcat Billy - Pygmy hippopotamus A wallaby Bruno - a black bear
And here's his lucky pet: Rebecca - Raccoon, intended for a Thanksgiving feast; First Lady Grace had a tree-house built for her instead.
I don't see any presidents with butterflies as pets. Here's more from the Butterfly Conservatory.