Single mailing campaigns can be sent immediately or easily scheduled to go out at a later date or time.
We're going to skip over the steps to creating and editing your single mailing, but for reference you can read through these steps in our article: How to Send a Single Mailing Campaign.
Select Schedule and hit Finish and you're all set.
And that's how to schedule a mailing. As always, if you have any questions, please don't hesitate to let us know. You can find all of our great support resources and how to get in touch with us via email, chat or phone on our Support Page.