Web Forms :: Adding A Control To A Calendar Control During DayRender?

I'm trying to do something that should be very simple, but getting completely stumped on why it's not working.

I have a page that has a few calendar controls, and when the user clicks a date, I want it to toggle the value (in this case it's the "availability" of the date).

I don't want it to post back with every calendar click, so what I'm doing is:

1. During the dayrender event, a Hidden Field gets added to the day. (As it is created, the Hidden Field is added to an array so that it can be retrieved later.)

2. When the user clicks, the value of the Hidden Field is toggled using JavaScript.

3. Once the user is done marking unavailable days, they click a Save button, which gets the values, and saves them to the database.

The Hidden Fields are getting added, and toggling is working just fine. The problem is that when the user clicks the Save button, all the Hidden Fields and their values get cleared, so at the time of the save, it thinks the length of the Hidden Field array is 0.

I think it has something to do with the order in whcih the page is rendered, and the "Save" code executed, but I'm not sure how to fix it. I'm doing something almost identical on another page that works fine - the only difference is that on that page there are no calendar controls, so instead of being added at dayrender, the dynamic controls are added on page load.

If I wanted to add a Hidden Field for each day in a calendar, whose value I can change with Javascript, how do I go about it in a way that I can access the values when clicking a Save button?