The First Saturday medal is awarded for attending Ingress First Saturday events. These are official Niantic-sanctioned events organized by Fev Games and run by local agents.
Check out the First Saturday registration page to learn when and where First Saturday events are occurring and register. For your participation to count, you must hack the registration portal during the event. The registration will have a triangular ornament around it, as pictured below:
Note that the medal does not retroactively apply to First Saturday events you may have attended before the medal’s release.
The First Saturday medal was introduced on 12 November 2018.