What's your actual business requirement? How does your business requirement define the same week in the previous month?
May 2016 has 5 Mondays, thus 5 weeks? How will you compare week 5 in May to a week in April which doesn't exist?
Sorry for late reply, extended weekend
I did not get accurate requirements from the customer. And it was my mistake to take the above approach.
I think the comparison to last month same week should be on the same dates only rather than what week they lie in?
What would your approach be in this matter? any suggestion?
'The best approach' depends on what you actually need.
I don't think this problem is a Tableau problem, it's a problem with your logic and requirements. Confirm what you need and we can help you build a calculation to achieve that. Right now it's a blank slate, and that's hard.