On the compose screen you choose Send now or Schedule. Scheduling lets you write the message when you have time and have it arrive when it's useful.
Scheduling one
Choose Schedule and pick a date and time. The time has to be at least a minute in the future.
Everything else works exactly as it does for an immediate send - same audience filters, same preview, same limits. See Sending a notification.
Once saved, the campaign appears in your Notifications list with a scheduled status and its send time.
Scheduled times are read in the server's timezone, not your local one and not your customers'.
If you're in a different timezone, work out the offset before you set the time - and if a scheduled send has ever landed at an unexpected hour, this is why. When in doubt, schedule a test to yourself a few minutes ahead and confirm when it actually arrives.
The audience is resolved at send time, not now
This is the most useful thing to understand about scheduling, and the least obvious.
Your filters are saved with the campaign, but they aren't turned into a list of people until the moment it sends. So anyone matching your filters at that moment receives it.
Schedule a message to everyone tagged vip for next Monday, and the VIPs you add on Friday get it too. Someone who leaves in the meantime doesn't.
That's usually exactly what you want. Just be aware the count you saw when composing is an estimate of a future audience, not a fixed list.
Cancelling
Open the campaign from the list and choose Cancel send. It won't go out.
You can only cancel while it's still scheduled. Once it's begun sending it's too late - the message is already reaching devices, and there's no unsend.
When scheduling earns its keep
Sending at a sensible hour. A notification at 3am gets seen at 8am with less impact and a little irritation. Schedule for when people are awake and receptive.
Timing around an event. The day before an event, the morning of a renewal, the start of a promotion.
Working when it suits you. Write Friday afternoon's message on Wednesday.
If you're building a sequence - a reminder a week out and another the day before - schedule both at the same sitting. It's easy to write the first and forget the second.
What happens when it fires
The campaign moves to queued, then sending, then sent, and its delivery report fills in as it goes. See Delivery reports.
Scheduled sends are picked up within a few minutes of their time rather than to the exact second. Don't schedule anything that has to land on a precise minute.