Text at top of Add Days window:
You have the option to add Additional Days. Simply select the Number of Additional Days below. The additional days will be automatically generated. They will be copied from the Event: same Title, Summary, Description, and Details as the Event. Later you can edit the Additional Days: required in order to change Start Date and Time, and End Date and Time such that they exist on different days than the Event (first day).
* Event Fees, Event Emails, Event Attendees are managed via Event. People signup via Event which includes Sessions, Breaks, and Days for the Event. All inclusive.
Boy, that's a mouthful.
I read through all that, then scroll to the bottom and find:
Cannot enter any more Days for this Event. All Event Days defined.
