You can compose an email message in Gmail now, let it stay there in the Drafts folder and have it sent automatically at a specified data and time with the help of Google Scripts.

The Schedule GMail script imports the draft message in a spreadsheet, you specify the send data and time and it creates corresponding time-driven triggers to automate delivery.

/* Written by Amit Agarwal of labnol.org */

function initialize() {
  /* Clear the current sheet */
  var sheet = SpreadsheetApp.getActiveSheet();
  sheet.getRange(2, 1, sheet.getLastRow() + 1, 5).clearContent();

  /* Delete all existing triggers */
  var triggers = ScriptApp.getProjectTriggers();
  for (var i = 0; i < triggers.length; i++) {
    if (triggers[i].getHandlerFunction() === 'sendMails') {
      ScriptApp.deleteTrigger(triggers[i]);
    }
  }

  /* Import Gmail Draft Messages into the Spreadsheet */
  var drafts = GmailApp.getDraftMessages();
  if (drafts.length > 0) {
    var rows = [];
    for (var i = 0; i < drafts.length; i++) {
      if (drafts[i].getTo() !== '') {
        rows.push([drafts[i].getId(), drafts[i].getTo(), drafts[i].getSubject(), '', '']);
      }
    }
    sheet.getRange(2, 1, rows.length, 5).setValues(rows);
  }
}

/* Create time-driven triggers based on Gmail send schedule */
function setSchedule() {
  var sheet = SpreadsheetApp.getActiveSheet();
  var data = sheet.getDataRange().getValues();
  var time = new Date().getTime();
  var code = [];
  for (var row in data) {
    if (row != 0) {
      var schedule = data[row][3];
      if (schedule !== '') {
        if (schedule.getTime() > time) {
          ScriptApp.newTrigger('sendMails')
            .timeBased()
            .at(schedule)
            .inTimezone(SpreadsheetApp.getActiveSpreadsheet().getSpreadsheetTimeZone())
            .create();
          code.push('Scheduled');
        } else {
          code.push('Date is in the past');
        }
      } else {
        code.push('Not Scheduled');
      }
    }
  }
  for (var i = 0; i < code.length; i++) {
    sheet.getRange('E' + (i + 2)).setValue(code[i]);
  }
}

function sendMails() {
  var sheet = SpreadsheetApp.getActiveSheet();
  var data = sheet.getDataRange().getValues();
  var time = new Date().getTime();
  for (var row = 1; row < data.length; row++) {
    if (data[row][4] == 'Scheduled') {
      var schedule = data[row][3];
      if (schedule != '' && schedule.getTime() <= time) {
        var message = GmailApp.getMessageById(data[row][0]);
        var body = message.getBody();
        var options = {
          cc: message.getCc(),
          bcc: message.getBcc(),
          htmlBody: body,
          replyTo: message.getReplyTo(),
          attachments: message.getAttachments(),
        };

        /* Send a copy of the draft message and move it to Gmail trash */
        GmailApp.sendEmail(message.getTo(), message.getSubject(), body, options);
        message.moveToTrash();
        sheet.getRange('E' + (row + 1)).setValue('Delivered');
      }
    }
  }
}