|Excel is evaluating the 4/5/2015 as a number, not a date.|
To prove this to yourself, click on the cell containing the formula. then click on the Formulas tab, then "Evaluate Formula". Each time you click the Evaluate button, you can watch Excel perform each evaluation step. Eventually you will see 04/05/2015 replaced with 0.000397022332506203, which is not a date.
Try this formula instead, so that Excel knows that you are looking for a date in C16:
Another option is to put a list of all of your holidays in a list then use ISNA and MATCH to see if the C16 Date is in that list.
The ISNA function checks to see if the MATCH function found a match. If the MATCH function returns #N/A, it means that no match was found and ISNA will return TRUE. The IF function will use the TRUE to return an empty cell.
Once again, you can use Evaluate Formula to follow the steps.
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