How to manage a range

I've 2 variables datetime

StartDate
EndDate

and I've x values

Campo1
Campo2
Campo3
Campo4
...

I would like to create a method that using a input parameter (month) find the value between Campo1, Campo2, Campo3... using the month of StartDate

example... If I've

15/09/2009 StartDate
20/04/2010 EndDate

my method parameters should be:

Test (StartDate, EndDate, Month) ... I think that I've to use also StartDate in my method ... I think

I've to calculate the number of Campo to read

if my input month is 10 (october) the output will be Campo2
if my input month is 1 (january) the output will be Campo5

if my input month is 4 (avril) the output will be Campo8
output variable will be a number (the number of the campo) and a null value if you can't find the month in the range (StarDate and EndDate)

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