Use the purge one-time-dates Holidays Tools to purge (delete) one-time-dates that are in the past and are no longer needed (or that you wish to replace by re-importing them from an Excel file).
To purge some of the one-time-dates of the currently-selected holidays set. click on the More... button that is located at the bottom of the list of one-time-dates in the Holidays Manager.
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Then select the Purge sub-menu from the popup menu that appears.