Do I have to choose either One-off or Regular in the category section? Yes, as this determines which calendar your event will appear in. 

How do I choose either One-off or Regular in the category section?  A One-off event is generally that and includes such things as workshops, fairs, retreats etc. Regular includes weekly, fortnightly, monthly or irregular but reoccurring events.

 If my event is a series over a specific time frame, is it considered a One-off or Regular event? If attendance is required at each and all sessions, then it is considered a One-off event. If people can attend any one or more in the series, then it is considered a regular event.

If my event runs regularly over a period of time, at this stage of our development you will need to enter the event each time. We are hoping in the future to be able to afford to make these events repeatable.