How do I schedule a send for later?

Short answer: In the send window, open “Deferred sending”, pick the date and time, then confirm. The send stays cancelable until it goes out.

Steps

Compose the email as usual from the Contacts page.
In the “Send Email” window, click “Deferred sending”. The date field appears — it is hidden by default so it does not clutter the ordinary send-now flow.
Fill in “Send date & time”. The time to enter is Central European Time (UTC+1 in winter, UTC+2 in summer).
The send button becomes “Schedule send”. Click it and confirm.

To go back to sending immediately, click “Cancel deferred sending”: the field folds away and the date is cleared.

Scheduling is open to everyone, and works for an emailing as well as for an email addressed to a single contact.

A scheduled send turns up in three places, and the cancel button is the same everywhere:

in the Send History of the Promotions page, at the “Scheduled” status;
at the top of the contact's detail sheet, as a banner;
for an email addressed to a single contact, that contact's row is also tinted in the list.

Click “Cancel scheduled send” and confirm. The send moves to the “Canceled” status and will not go out.

A scheduled send is canceled, not deleted. The delete button is in fact hidden while a send sits at the “Scheduled” status.

You'll know it worked when: The send appears at the “Scheduled” status with its date, and no recipient has been served yet.