A Perfect Number

There is a hidden gem that comes
With a three day weekend, and it sums
To be that since there are seven days,
Seven minus three comes out in only one ways.

As any mathematician will tell:
Seven minus three can be quite well.
In fact, it is a perfect square,
Which a carpenter could use for a perfect chair.

And not only that, but it's root is prime!
Which makes it odd most of the time.
Only this prime is special, as it's even
And balanced like a table during the feast of Stephen.

Though, some mathematicians might want to play:
"It is five plus e to the i pi," (or Euler's identity).
But that's simply four, and they're being a jerk
To tell you that you've got one less day of work.