If you want the code to repeat all day, you can change the macro to this:
Dim sht As Worksheet
Do 'creates an infinite loop'
For Each sht In ActiveWorkbook.Sheets 'loops through each sheet'
sht.Activate 'activates each sheet'
Application.Wait (Now + TimeValue("00:00:20")) 'waits 20 seconds before going to next sheet'
This creates an infinite loop that steps through each sheet every 20 seconds and starts over once it gets through them. I've added some comments for your own curiosity, but you don't have to change anything to make it work.