Tomorrow is 6-6-6 (if you ignore zeros). Here is the entry for 666 in David Wells’s Dictionary of Curious and Interesting Numbers.
666
The 36th triangular number (666 = 1/2 x 36 x 37) and the Number of the Beast in the Book of Revelation: ‘Here is wisdom. Let him that hath understanding count the number of the beast; for it is the number of a man, and his number is six hundred, three score and six.’
A number beloved of occultists, who throughout the ages have used gematria to find the Number of the Beast in the names of their enemies, political or theological. The fact that some ancient authorities give the number as 616 has not deterred tham. With a little ingenuity, both numbers can be found instead of just one.
Peter Bungus made Luther equal to 666, by using the old system, which counts A-I as 1-9, K-S as 10-90, and T-Z as 100-500. Bungus read Luther’s name as Martin Luthera, half German and half Latin, a typical bit of skulduggery, but Bungus was an expert. He wrote a dictionary of numerological symbolism.
666 in Roman numerals is DCLXVI, which uses each letter under M (1000) once, which has led to the suggestion that this is the origin of 666. It could merely be a way of expressing some large, or vague, number.
-2 sin(666) is a good approximation to the Golden Ratio, phi.
phi(666) = 216 = 6 x 6 x 6
[Note from Derb: Confusingly, Wells is using phi in two difference and unconnected ways here. First, he’s using it to refer to the Golden Ratio 1.61803398874989484820458683436563811772…, a.k.a. (Sqr(5)+1)/2. Second, he’s using it to refer to the Euler Totient function, which for any positive integer N is defined to be the number of positive integers not exceeding and relatively prime to N. phi(15), for example, is eight: 1, 2, 4, 7, 8, 11, 13, 14. Note you are allowed to count 1, but not (illogically, but it makes the downstream math simpler) N. Note also that in regard to -2 sin(666) being a “good approximation” to phi, Wells forgets to say that he means 666 degrees, not radians; and it isn’t just a good approximation, it’s precisely equal, because of some well-known properties of sin(k*pi/10) for whole numbers k. But never mind all that, here’s the one I really like….]
666 = 2^2 + 3^2 + 5^2 + 7^2 + 11^2 + 13^2 + 17^2, the sum of the squares of the first seven primes.