The emails on Gmail can be scheduled using its "Gmail Delay Send" service. Please visit the link provided, to visit Google's help page which is self explanatory on how to use this service. The article also contains a youtube video to help you further.
Please follow the steps mentioned and let us know in case you face any difficulty.
I use a Google Sheet which is capable to deliver message at scheduled time. It has a macro which take care of sending message at scheduled time. You need to fill the details in the sheet like To, subject, time and date of delivery. To download the sheet login with your Google account and use the below link.
Link - http://goo.gl/eUXtGT
When you open this link you will be asked to 'make a copy'. After clicking the link a copy of the Google sheet will be stored in your Google Drive. You will find a new item in toolbar as Gmail Scheduler. You need to authorize it first. Then you can write mails and set the time you need to send. It looks just like excel sheet. By looking at the attachment you can figure out how it works.