It's easy afterward to customized and get any look you want.
Below I opted for a "grunge" look, therefor I used the Downcome font then added some textures, some dirty spots and then applied a "Tear off" effect (that you can find here)
Here's below January 2011 which looks like the it has been printed on potatoe's sack.
There's a similar method for Photoshop you can find at : http://psd.tutsplus.com/tutorials/designing-tutorials/create-a-calendar-using-scripting-in-photoshop/
Originally found on 100 Best Photoshop Tutorials of All Time that Yield Professional Results
