You can schedule the date and time of when an email will be sent by selecting the Schedule Send option.
The scheduled email will appear in the Comms and Timeline tabs, along with who scheduled it and when it has been scheduled for. If the email has been scheduled to send more than 2 minutes later, a 'Send Now' and 'Cancel' button will appear to let you send the scheduled email straight away if you have changed your mind or to cancel sending the scheduled email altogether.
Note that you cannot schedule an email that exceeds the size set in Builder - see here for more information. However, validation of the email size occurs as part of saving the email, not at the sent time. So if a 25MB email has been scheduled and then the maximum email size gets reduced in Builder to 10MB, the scheduled email will still be sent as it conformed to the size allowance configured when it was scheduled.