You have a circular ref err (formula in A3):
=LOOKUP(2,1/('Period Dates'!C5:C17<=A3)/('Period Dates'!E5:E17>=A3),'Period Dates'!F5:F17)

Should be:
=LOOKUP(2,1/('Period Dates'!$C$5:$C$17<=$B3)/('Period Dates'!$E$5:$E$17>=$B3),'Period Dates'!$F$5:$F$17)