Can I publish an OS X Server hosted calendar?

There are a couple of things you would need to do.

1. Open your router to allow calendar traffic in and out of your network (be carefull about opeing any custom ports on a router as it could allow for unotherised attacks). All internal user passwords should then be strong and all traffic should be encrypted.
2. Have your external server.mydomain.com DNS point to a public ip address. - At your router you can rout it to the server machine (public ip's would be supplied by your ISP). This will allow external calendar subscription traffic know where to get the information from.
3. For invetation replies to work from outside your network to your internal network you will need to get your ISP to set up reverse pointer for your domain to point to your public ip address. This allows people outside your network to accept or decline your calendar invetation - and allows your calendar server to automatlycally post those replies in the relevent calendar.

I havnt gone into much depth with these steps because personally I have moved to using iCloud.

For all the hassle and potential risks of opening up your network I would alternativly set your users up with a free iCloud account and use the calendar service there. Super easy and it works. It also limits the potential external attacks on your network.

Try this:

http://support.apple.com/kb/HT4777

Good luck.