A nice and useful collection for calendar icon making.

To design something unique you might find useful some related tutorials.

Calendar icons not only for your blog
Wit CSS-code and a source image for GIMP.

Web 2.0 Date Icons For Your Blog
png-format with a .psd-source.

Calendar Icon Tutorial

Calendar Icon

Calendar icon – Photoshop tutorial

Creating a Blog Entry Date Calendar Icon Look with CSS, Mostly