Tab number in COUNTIF calculation

Hi

Need to collect data per week [each in a separate tab] and count how many occurances are there into a 

summary tab using COUNTIF 

Cell X = =COUNTIF(Week20!$D$5:$D$40,Week10!$D$1)

How can I replace the 'Week20' in this example with the content of cell Z2 , that contains the name of the tab (week no.) it relates to