The all-day events are displayed at the top of each day and highlighted with a color associated with a calendar the event is added to.
To change some all-day event settings proceed as for a single event. Click the all-day event title and use the Edit button at the bottom of the pop-up window to start editing.
To apply the changes you made use the Save button.
You can also change the all-day event end date right in your calendar without opening the editing window as described above.
Place the mouse cursor at the right boundary of the event box. The cursor will turn into a bidirectional arrow. Then, without releasing the mouse button, drag it to change the event end date.