Javascript Calendar
First, When I mention a javascript calendar, I mean a javascript event calendar. I do not mean a javascript date picker. An event calendar is calendar used to display events or a schedule. That is the primary difference, since a date picker will not allow you schedule events.
Next, an event calendar by definition contains events. The events can be thought of as data for the calendar. The new standard for using data on the internet is xml. It is useful because a complex set of relational data can stored in a single string (or piece of character text. Xml can be found in both AJAX and SOAP technologies, and is widely used.
A javascript calendar should also maintain a clear seperation of event data and display. Again, xml is a good choice to use as a data source since it will help maintain the seperation. Plus, display can be held in an entirely seperate XLST file.
Last, you'll want a javascript calendar that is cross browser compliant. In my experience, most people are using Microsoft Internet Explorer these days. Firefox is a popular alternatively to MSIE. Plus you have Netscape and Opera browsers. For Apple MACs, you commonly see Safari replacing the older MAC IE.