Open up a compose window in Gmail and write your desired message as you normally would. Instead of sending, click the green ‘Later’ button at the bottom of the screen and choose the date that you’d like for your message to be sent.
Once you create your scheduled email, it will be saved to the Scheduled section of the Events Dashboard in Gmail. If you'd like to edit the date or time, you may do so from the Events Dashboard.
If you'd like to cancel the scheduled email, simply locate the message in the Scheduled section of the Events Dashboard in Gmail. From there, click on "unschedule" or modify the scheduled send time.