The only requirement for creating an event is that it have a name. In the context of importing constituents to an event, you need to have a value in the "Gen. Event Name" column/field.
NOTE: When loading gift records, you can associate gifts with events by using the "Gift Event Name" field.
Custom categories and attributes (different types of fields) can be added to an event in LGL. During import, those custom fields will be available to you if you choose to display them and choose the event.
NOTE: If you want to import to these custom fields, your import file can be imported only to that one event.
It is also possible to have recurring events in LGL. Read the Recurring events or classes - how to record attendance Knowledge Base article for more information.