After creating your email campaign [You can find how to do this by going to 'How do I start an email campaign' article in the sending campaigns section] you will see a few options: 

To schedule a campaign you'll want to select the Schedule a Campaign option.

You should now have a pop up window that requires the date and time to send the email.

 After you have confirmed the message is what you'd like and sent a test to yourself [Not required but reccommended] you can now Schedule this campaign!