Jaime
Here is one way.
=VLOOKUP(MONTH(A1),{1,"Sheet1!A1:A10";6,"Sheet2!D1:D10"},2,1)
where an array constant is used as a lookup table. You can expand this table at will [ If you need more help on this concept, just ask. ]
And name the cell of the formula via the Name Box.
Lets say it's named as ThisSheet.
You need to invoke INDIRECT(ThisSheet) in formulas in order to access the range ThisSheet really refers to.
Aladin
Yes; First name the two ranges individually (Ex Rng1 & Rng2). Then choose Insert>Name>Define and create a range name (Ex YourRange). Then in the refers to field enter =IF(Month<7,Rng1,Rng2) where Month is the name of the range where the month value resides. Now to toggle based on the month value, chosed Edit>Go To>and enter YourRange in the reference box. You can also use a hyperlink to YourRange or a command button.