Sheet Reference (Variable) in Formula

I have a spreadsheet that has each month on a tab. The current month beginning balance is the previous month ending balance. Rather than having to type the sheet name into each beginning balance formula, I would like to type the sheet name into one cell on the spreadsheet and all of the formulas refer to the single cell for the sheet name. Something like ="C2"!F21 and ="c2"!F22 etc where C2 has the name of the previous months sheet name (JAN).