Normally, posts are published live when you click on the blue “Publish” button.
You can however modify the publish date both backwards and forwards in time. If you set a publish date into the future, the post will automatically be published on that date and time.
To modify the publish date, click on the date or the world immediately, next to the “Publish” heading in the “Status & Visibility” section in the Document tab of the Inspector.
From the dropdown that appears you can choose any date, and time.