This is an overview of what fields are required and what fields can be empty when entering events and to-dos:
In the General Tab, you should at least fill the Title field and the description (the large rectangle), even though neither is mandatory. If you set a Reminder, you should also specify the time, and either the sound to be played or the program to be run.
The Attendees tab may be left completely empty.
The Recurrence tab must only be filled if you select a daily, weekly, monthly or yearly repeat.
The Attachments dialog may be left completely empty.