This year, 24 Hour Comics Day will be October 20, 2007. (It’s moved from April to put it further away from Free Comic Book Day.) Watch for more information later in the year about which locations will be holding events.
Update: In the interests of providing greater context to new readers, the 24 Hour Comic is an artistic exercise created by Scott McCloud in which you attempt to create 24 pages of comics in 24 hours. 24 Hour Comics Day is a semi-official gathering in which you do this in groups of people; it often takes place in comic shops.