And if you are part of the Church of Bob or the Cult of the Sub-Genius then today is Saint Caligula Day.

@sean-law , I can't believe I've never heard of Yellow Pig day. That's some cool stuff.

It is the only known prime that is equal to the sum of digits of its cube (17^3 = 4913, and 4 + 9 + 1 + 3 = 17)

17 is the only positive Genocchi number that is prime.

There are exactly 17 two-dimensional space plane symmetry groups.

It is the only prime that is the average of two consecutive Fibonacci numbers.

There are exactly 17 ways to express 17 as the sum of one or more primes

In the Irregularity of distributions problem, consider a sequence of real numbers between 0 and 1 such that the first two lie in different halves of this interval, the first three in different thirds, and so forth. The maximum possible length of such a sequence is 17.

17 is the minimum possible number of givens for a sudoku puzzle with a unique solution.

There are 17 orthogonal curvilinear coordinate systems (to within a conformal symmetry) in which the 3-variable Laplace equation can be solved using the separation of variables technique.

17 is the first number that can be written as the sum of a positive cube and a positive square in two different ways; that is, the smallest n such that x^3 + y^2 = n has two different solutions for x and y positive integers.

17 is the minimum number of vertices on a graph such that, if the edges are coloured with 3 different colours, there is bound to be a monochromatic triangle. - Ramsey's Theorem.

and there are many more...