A basic Calendar is provided by Celtx. It allows users to create a project calendar and schedule dates and tasks that can be shared with team members (via a WebDav folder). Users can create calendars on a per project basis, then filter the calendar view for a particular project, or view all of them to get a sense of upcoming dates and deadlines.
You can also keep multiple calendars in a Celtx Project, each of which will now be included in the Collaboration sync, allowing you to share calendars with team members when you Upload your project to the server.
Source(s): Calendar Features support
Adding a Calendar
Any number of calendars can be added to an open project by first selecting the 'Add Item' icon located in the Project Library Toolbar. Then select 'Calendar' from the Add Item Dialog, name it and hit OK. The new calendar will appear at the bottom of the Project Library. Select and double-click to open.
Deleting a Calendar
We’ve also switched to Lightning for our Calendar. The Mozilla Lightning Project uses the same code base as the Mozilla Sunbird Project, which was the previous calendar we used, so your dates should transfer over nicely.
The Lightning development team keeps a development weblog. It is intended for everyone who is looking for in-depth, up-to-date information about current events in Lightning development: