Hold down the Shift key while right-clicking the tabs (worksheets) you want to search in (or, if you need to search in tabs that are not adjacent to one another hold, down the Ctrl key instead).
Then press Ctrl+F to bring up the search applet. Type in the date (as it appears in the cells), & click on Find All. You'll get the total number of times it appears in all the tabs in the status bar at below left.
Here's an example screenshot click here - I searched for something across 5 tabs & it turned out '283 cell(s) found' in the status bar. So the thing I was looking for appears 283 times. G