You will have to setup a list of Public and other Holidays
The , is not a range, the array is a list of individual dates
It is probably limited at about 250 characters, which should be plenty for your needs
You can't have the dates in another cell, but you can use a Named Range to store the dates
so =+NETWORKDAYS(B5,AA5,PHolidays)
is valid if you have a Named Range PHolidays which is equal to ={"4/1/2010","6/1/2010"}