Having missed 'Talk Like a Pirate Day', I vowed not to miss Mole Day! Mole is 6.02 x 10^23. I'm no math-head and I can barely understand what that means, except that it's a big number and mathematicians use it to calculate big things. Mole Day is (for obvious reasons) October 23 from 6:02 am to 6:02 pm -- that much I can understand.
To celebrate the occasion, here's a cheerful, geeky video for you:
Happy Mole Day to You!