Select a release date for the item by clicking on the calendar icon.
If one of the site editors has the responsible button ticked on their permissions settings, you can also set a Notification Interval to send an email every x days since the item was last modified (you also need to tick the Enable Notification box). This is a way of reviewing old content.
(can only be done with news items)
At the bottom of the news item edit screen, click on the new button to activate the release date and/or expiry date:
Click on the calendar icon to select the release/expiry date:
Save and publish the file.