You can now schedule your emails in Gmail to be sent at a later date and time.
To schedule the sending of a message:
Compose your message as normal
If you don't want to send the message immediately, click on the drop-down arrow of the Send
Choose Schedule Send
Choose a suggested time to send the message
Alternatively, click on Select date and time to choose a specific date and time to send the message
The message will be moved to the Scheduled folder from where it will be sent at the selected date and time. To cancel the send, click on the message in the Scheduled folder and click on the Cancel send link.