Pages may be created but scheduled to be published at some later date. Or you can schedule a page to expire at a certain time. These features are most useful for news or announcement pages but can be used any time you want to create content but add or remove it from your site at a specific later date.
The publishing schedule is attached to a specific revision so in theory you could schedule a first revision to publish tomorrow - and then schedule a second revision to publish on the day after. If you are interested in this sort of functionality, you can read the details in the Wagtail documentation. But for most purposes, the first step should be create a final draft of the page.
Once you have your final draft, click the "i" icon in the upper right and click the words "Set schedule". Fill in the date and time you want this item to be published and click "Save Schedule".
Note: This publishing schedule will only take effect after you have published. So when you are satisfied with the content and the publishing schedule, publish your page. If you choose "Save draft", your page will not be published at the requested time.
When you have scheduled publishing for a later date, the page will be locked and any attempt to edit it will display the red banner shown below. You will need to cancel your scheduled publishing in order to make any corrections to the page. The page retains the publishing time you had selected, so when you publish your changes, the original schedule will once again go into effect. If you do not see the red banner and the scheduling information still says "Once published", you have not published your schedule and it will have no effect.
Occasionally you may want pages to be removed from your site at a specific time - for example once an event has taken place, you may want to automatically remove the page displaying the event registration form.
Page expiration is controlled by the same "Set Schedule" popup you use for scheduled publishing. Choose your expiration date and then publish the page. Unlike publishing pages, expiring pages does not lock the page and the rules around expiring pages are a bit complicated, so I recommend only setting an expiration date on the latest revision of a page.