Special properties of number 15 (Today’s Date)

1. 15 is a triangular number  as 15 = 1 + 2 + 3 + 4 + 5
2. 15 is a hexagonal number
3. 15 is a palindromic number in binary system (1111)
4. 15 is the magic constant for a 3×3 magic square, the smallest possible nontrivial magic square. An N×N magic square consists of the N2 integers from 1 to N arranged in an N×N square grid, such that the sum of any row, or any column, or of one of the two diagonals, is equal to any other. There is only one solution for a 3×3 square, not counting its rotations and reflections:
  4  9  2
  3  5  7
  8  1  6
This magic square was known to the Chinese at least 3000 years ago and is called the Lo Shu.
5. The Chemical Element Phosphorus has an atomic number of 15.
6. The only 15-letter word that can be spelled without repeating a letter is "uncopyrightable".