Excel lookup on multiple rows and columns

[zucchini123]

Hi, i try to build a table which contains rows with a list of name (A,B,C,...aa,bb,cc..etc) and columns as every Sun of 2009

Requirement: Cell for 'A' and cell for 'aa' cant have data at the same date (column), same goes for B to bb..C to cc...etc

xx '1st Sun' '2nd Sun' '3rd Sun' '4th Sun' ............
A data
B
C data
aa data
bb
cc data

Is there any way to use vlookup/hlookup to verify, (what i want to do is to put it into the data vertification, so once i had data in A "1st Sun", and will block me insert data to aa "1st Sun")

Thanks.