Desktop email programs like Outlook have an email scheduling feature that you can use to send emails at a preferred time. You can compose a message now, it stays in your outbox and is automatically delivered at the set date and time.
Boomerang is a browser extension for Gmail (and Google Apps for Gmail) that brings similar “Write Now, Send Later” feature to your Gmail.
Just compose your email message and then use the new “Send Later” button in the Gmail compose window (instead of Send). Choose the exact date and time that the email is to be sent and you can log out.
Boomerang will automatically deliver the message at the desired time. The free account lets you send 10 messages per month.
Right Inbox is another alternative to Boomerang with a similar sending limit for free accounts.