Hi,
try this (sorry for any translation mistakes, I use a Swedish Excel version).

Put some dates in column AE (for example). Lets say you reserve AE2:AE99 for this. Enter 1st of June 2011 (in your date format) in cell AE2 (just for test purpose later)

Mark cells C12:Y44. Select menu "Start / Conditional formatting / Create new rule". Select the last alternative "Determine what cells should be formatted using a formula". Enter this formula:
=IF(VLOOKUP(C12,$AE$2:$AE$99,1,FALSE())=C12,TRUE() )
and select an ugly colour to highlight your special dates.

Press OK and now you should see 1st of June highlighted

Kind regards
Bjorn