From the Holidays screen, click the name of the Holiday you want to edit. The Holiday details will show up in the "Details" section.
If you want to edit the Name of the Holiday, click the Name field and enter a new name for your holiday
If you want to change the day/month of your Holiday, use the interactive Calendar to select a new day (or range of days) for your Holiday.
If your Holiday repeats annually, click the "Repeats Annually" slider so that the slider moves to the right and turns blue . Click the slider again to disable "Repeats Annually" .
After making changes to a Holiday, click the Save button to store the changes.