What is iCal Day you ask?
The icon for the Mac application iCal is a depiction of a little desk calendar, set to July 17th, which happens to be the date when iCal was first introduced to the world at Macworld Expo in New York City in 2002, by Steve Jobs.
When iCal is launched, its icon in the Dock automatically changes to display the current date. So July 17th celebrates the only day of the year where iCal’s icon doesn’t have to update. It displays the correct date even when the application is closed. As far as I can tell, the first one to recognize this marvelous day was Steven Frank, who refers to it as iCal Synchronicity Day.