Zuzana,
You need something like:
=VLOOKUP($A2, INDIRECT("'[New format "&B$1&".xls]sheetname'!A:B"), 2,
0)

The INDIRECT() function builds dynamic addresses. $A2 is "dollared" so
that the lookup value is always in column A:A. B$1 is dollared so that
the date is always in row 1:1. Hopefully all referenced workbooks will
have the data in a sheet named identically. Otherwise you might need to
have a nested VLOOKUP(), with the sheet names per date.

Write back if you need details.

HTH
Kostis Vezerides