I want to disable my flow Next button if not selected check box , if selected check box then need to enable next button in flow screen is it possible?

Based on check box value disable or enable Flow next button please help on this topic 

I am a bit of a beginner at formulas, so hopefully one of you will think this is easy, and be able to help. :-)

 

Customer requirement:

 

Create a formula field that displays the Week Number (as Week 1, Week 2, etc), based on the Date entered on a particular record (same object).

 

Ex: 01/01/09 - 01/04/09 would return a value of "Week 1"  

01/05/09 - 01/11/09 = Week 2

01/12/09 - 01/18/09 = Week 3

 

and so on.....

 

Is using the CASE function the way to go, and if so, what's the shortest way to create this formula?

 

Thank you in advance!

S