Today is a mathematically perfect day. A perfect number is one that is equal to the sum of its (proper) factors.
For example, the factors of 6 are 1, 2 and 3. Add these up and you get 6.
The factors of 28 are 1, 2, 4, 7, and 14. Add these up, you have a perfect 28.
So every year, 28/6 (28th of June, or 6/28 for my backwards American friends) is a Mathematically Perfect Day.
There’s not many of these perfect numbers, and the ones we have discovered get very large very quickly. For example, the next number is 496, then 8128, then 33 million.