Changing Icon per Day

Just like the Calendar app on the iphones and ipods, how do I make the Icon change per day?

I assume this is for an iOS app. The answers is, you can't. The Calendar app has access to functionality in iOS that us mere mortal developers cannot access. You may be able to pull this off with a jailbroken phone, but I've never tried that.

Since changing the icon is not officially supported, if it's numerical information you want to show, you can use the Notification Badge instead.

Updating iOS badge without push notifications

So you could make it show today's date in the badge, and schedule the badge to update at midnight, or show the current temperature and schedule it to update hourly.