Ada Lovelace Day (it changes every year but seems to be the second Tuesday of October...)
HPL Day (20th of August I think, or March 15th depending on which anniversary we celebrate)
To go with Pi Day, we could also celebrate Pi Approximation Day on 22nd July.
Anyone else feeling like we're turning into Google?
To perhaps resolve the Lovecraft problem why don't we celebrate 22nd March - anniversary of R'lyeh Rising! Then it is mythos focused rather than on any particular persons!