Looking for Online Calendar Software

I am looking for a form of calendar software which visitors can use.

For example, lets say you ran a holiday website. Visitors to the site can plan out there future holidays within their personal online calendar. Recording where they would like to go and when.

Does anyone know of any software such as this?

I’m not sure how, but i guess you could use google calendar API?

Off the top of my head I’ve seen a setup with drupal and a calendar module that can do something similar to what you are talking about though you might have to do some tweaks.