You could use the volatile INDIRECT function to refer to arbitrary worksheets, e.g., with a given date in cell C5, =INDIRECT(TEXT($C$5,"'dd mmm'!")&CELL("Address",A1)) would refer to the A1 cell in the worksheet given by C5. However, if you have A LOT of formulas calling INDIRECT, you can really slow down Excel's recalculation.
If there are worksheets for each day of the year, and if they're all contiguous with '01 Jan' leftmost and '31 Dec' rightmost, you could use a defined name and a non-volatile user-defined function to access any of those worksheets. The defined name would be _ALL_ referring to the formula =COUNTA('01 Jan:31 Dec'!$1:$1048576). The user-defined function would be
Call this as =ref3D(DATE(2020,10,15),A1,_ALL_) in cell formulas.
Formulas calling ref3d with _ALL_ as 3rd argument become dependent on all cells in worksheets '01 Jan' through '31 Dec', so any changes in those worksheets would be reflected in formulas calling ref3D this way when Calculation is set to Automatic, but changes in other worksheets would leave these formulas unaffected. Note that the 3rd argument to ref3D isn't used in the body of the function. That's intentional. It's presence as 3rd argument when called from cell formulas is all that's needed to make those formulas depend on all cells in worksheets '01 Jan' through '31 Dec'.
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