Creating calendar subscriptions

Question

Hey all,

I was wondering how to make calendar subscriptions that will automatically update in your calendar client as events are added. A lot of people have a Holiday subscription that will pull down popular holidays, how do you do that on that on the backend?

Unlike the holiday calendar, I would like it to be able to add and remove events automatically (doesn't have to be pushed)

If there is a difference in the implementation for different calendar clients, I would like to be able to use it with Apple's iCal.

Thanks! Matt