The nice thing about UNIX is that you aren't constrained simply be being forced to run a local stand-alone client. You can be the enterprise on your own little box.
I've provided a couple of options for you to peruse, that should be able to more than satisfy your need for a robust calendar capability, growing and expanding as you like:
is only 16 MBytes, and runs on any LAMP or WAMP system.
Also, another one of my favs is: http://vtiger.com/
coming in at 62 Mbytes and is also LAMP based.
I've included screenshots of both system's calendars too, for your convenience. Very full featured, will do practically anything you want them to.