Using Thunderbird and Lightning it's possible to have multiple calendars. You can create these in Calendar Mode or in Task Mode.
Next, you get to choose whether you want to create the calendar "on your computer" or "on the network".
When choosing "On my computer", you can only use the calendar on the computer you're working at. All the data is stored inside an internal database (the sqlite file storage.sdb in your Thunderbird profile).
When choosing "On the Network", your files can reside on general webservers (read-only), on WebDAV servers, on specialized CalDAV or WCAP servers, on the local filesystem or a network share, on FTP servers, or you could use your Google calendars.
Next, you have to choose a format of your new calendar as follows.
You're able to subscribe to calendars provided by third parties, mostly in ics format. These are almost always read-only; you won't be able to change anything for the calendars. An example of this kind of calendar are the holiday calendars provided by Mozilla.