When you see this message, it means that as the Liturgical calendar is being calculated, a day spot that should be empty is not. The way that this error message is generated is interesting because it only appears when calculating calendars for leap years that occur on a 400 year boundary (1600, 2000, 2400, etc...) and ONLY if you click on the Lookup two or more times in a row (i.e. Without clicking on the GO button or canceling the dialog box). You can reset the program by closing it and then reopening it or go through the calendar tab to view the days available you wish to view.