Next Sunday is the 10th October 2010. This is a date which is quite important with regards to life, the universe and everything because it is 10/10/10. 101010 is binary for the number 42.
As a result, H2G2 fans are declaring 10th October 2010 to be "42 Day". A list of events around the world can be found at http://fortytwoday.com/