There are four fields required in order to be able to save an event. When these fields aren't filled out, the 'Save' button is grayed out and you are unable to click it.
The required fields are...
- Start Date/Time
- End Date/Time
Note on locations
If you are creating multiple events in a row, sometimes the location from your previously saved event will look like it has been selected for your new event when you haven't selected that location for specifically that event. When this is the case, the 'Save' button is still grayed out because a location hasn't actually been selected for that event.
If you click the 'Location' button and reselect the location you want to add to the event, the 'Save' button will become clickable, and you can save your event.