It isn't currently possible to queue messages to be sent at a later time or date. At the time you want to send the message, you'll need to set it up and send it. You can only have one of each communication type in draft at a time.
This is the same for all methods of sending communications through Arbor to staff, students or guardians or parents, including mail merge.
If you'd like us to develop this feature in the future, please submit your feedback: Our Product Roadmap and how to submit feedback