A calendar that I really liked and has inspired me to include this into my site is 30boxes.com.
The calendar is built using mostly ajax and php and I'm looking to have something similar to it where people can enter simple sentence patterns and the calendar understands them as commands to post the event, when, and whether it should repeat or not.
This calendar will be in a social networking site built with php/mysql so the calendar must be able to interact with the friends feature. For example, it will need to show if two friends are going to the same event.
Overview of functions:
1. Has multiple views (day, week, month)
2. Be linked to weather sites to get forecasts for the day
3. User can click on a day and post their event
4. Be able to send emails to people to notify them of their event.
(user sets whether they want the reminder one day before, 1 week before, etc)
5. when you post an event, you can send invitations to people on the site
6. When you are filling in details for the event, it should be in pop-ups.