Some dates have a special meaning for any of us, right ? Friday13 is a tweak which replaces the current date displayed on the SpringBoard calendar icon with a date of your choice. Friday the 13th is just the default date setting, you can change it from Settings.app, as well as the date format and the displayed colors
For any suggestions and/or ideas for improvement, feel free to contact me via mail or twitter :-) (twitter link is below)
Configure icon options from Settings app.
See more Screenshots below