Is your Month field is in actual date format??
Yes the Month Field is in Actual Date format.
Tried Something , it will fill your requirement , but not most optimized solution.
Create a calculation like bleow.
lets name this calculation as cal1
Use this calculation as your measure. The restriction with this method is if your having having only 1 continuous Null value it works fine. If you are having max 2 continuous Null value you need to create 2 calculations like wise if you have max 3 continuous Null values you have to create 3 calculations. as you are having only 12 values in your requirement you can create 11 calculations and use last calculation as measure.
Cal 2 will be like this :
Only change with the previous calculation is we will change the filed as previous calculation (Highlighted in Bold)
so Cal 3 will be like this
like this you can do.
If we could have Circular Reference allowed we could have done this with only one calculation.
Hope someone else will provide better solution.
Thank you praveen