## Happy Eulerian day!

Submitted by Rachel on November 26, 2010Worried you missed Pi day? Never fear! Thanks to the kind people of Wolfram we now have a bevy of mathematical dates to celebrate — six in November alone! November 23, or 11/23 for people in the US, is Fibonacci day as 1,1,2,3 is the start of the Fibonacci sequence. And Twilight fans will be excited that recent months have not only included a Vampire day on 5th of October (since 10052010 = 5001 × 2010), but also that 25th of September was a Cullen day (as 25 and 9 are Cullen numbers)!

Today — 26th of November, or for Americans like the people at Wolfram, 11/26 — is a Eulerian day. The Eulerian number, *A(n,k)*, is the number of ways you can rearrange the integers {1,...,n} so that your permutation is made up of exactly k increasing runs of numbers. And 11 and 26 are the 4th and 5th Eulerian numbers when k is 2 — reason enough to celebrate for *Plus*! We're off to the pub!

You find out all the mathematical dates we have to celebrate in the Wolfram Blog: Happy Vampire Day.