For discounts and automated email scheduling purposes, a gallery is officially marked as "active" when either of the following triggering events occur:
A gallery containing two or more images is manually given the status of "Allow Visitor Access."
A second image is uploaded to a gallery that was previously given the "Allow Visitor Access" status.
The reason that the active date is officially recorded after the second image has been uploaded is so that you can upload a featured image on galleries that are created and displayed ahead of time, without the risk of triggering sales or automated emails before a gallery is actually ready to go.
Where the Setting Is Found
The Allow Visitor Access setting can be found on either the initial "Create Event" pop-up window, or within the gallery settings after the gallery has been created.