After switching the dropdown to public, you'll gain two new fields.
Public Sale Start: This is to set a future on-sale date and time.
Public Sale End: This is to set a future sale closing date and time.
If you don't set a sale closing time, online sales will close automatically at the time set in your Department Settings -- typically 15 minutes prior to the event. This does not affect the ability to sell in the Point of Sale tool.