Where depends on the version of Outlook. In Outlook 2007 it's the Delay
Delivery button in More Options in the Options ribbon. Search Outlook Help
if you have another version.

Be advised, though, that if you compose a message at, say, 12:00 noon and set
it not to send until 4:00 PM, the recipient will still see that you composed
it at 12:00 noon.  If this is an issue, you will need to find another way to
automatically send the message at a specified time and have it reflect the
actual sent time.  There are one or two 3rd party add-ins at
http://www.slipstick.com/addins/special_function.asp that will do this, such
as Mapilab's E-mail Scheduler.