The date you are adding is one which is in the list of years for which there are no occurrences
This message occurs when editing the list of Recurring Holidays Exceptions, if you are adding a specific date (at the top of the dialog) which is in the same year as one of the years that is listed (at the bottom of the dialog) as having no holidays occurrence at all..
Clearly this is inconsistent : you cannot tell Q++ (at the bottom) that the selected holiday does not occur in 2003 (for example) and then add the date of February 2, 2003 (at the top). This message is there to let you decide which of the 2 options to choose.
If you click on Yes, then the specific date will be added to the top list and the year of that date will be removed from the bottom list. If you click Cancel then nothing will happen : the specific date will not be aded to the top list.