A company delcared a dividend with an ex-dividend date of 7/22/09 and will pay the dividend to shareholders of record on the close of business on 7/24/09. Do you have to own the stock through 7/24/09 or can you sell it on the ex-dividend date?

"... on the close of business on 7/24/09." Doesn't that kind of already answer the question?

Besides, why not keep it one extra day? (OK, three more days since that's a weekend.)

To get the dividend you must own the stock as of the close of business on the day before the ex-dividend date. So in your case you can sell on 7/22 (or later) and you will get the dividend. The 7/24 date is immaterial in this. For most companies the x-div date is typically a couple of weeks before the date they send out the checks.

Ha - I totally mis-read that. I didn't even see two dates. Sheesh. :-)