All-day Events are the Events on your Calendar that do not have a designated start or end time. Unlike when you schedule an Event, all-day Events require that you pick a day that the Event takes place and not a time.
Since all-day events do not have a designated start time, by default they will appear above any Events that do have a start time on your Calendar. However, using the “Customize” screen, you can also choose to have your all-day Events appear below any Events with a start time. See the below image for an example:
To change the position of your all-day Events on your Calendar, follow the steps in the image below: