New Unannounced Event: Legendary Dinner May 1, 2019
On May 1, 2019, New Zealand reported that Tier 5 Eggs spawned at every available gym at 6PM. This behavior is exactly what happens during the Legendary Lunch Hour Niantic announced earlier this year.
Groudon is the current Tier 5 boss, which will be available for 1 hour starting at 6PM local time. Eggs will begin to spawn 10 minutes prior to the event taking place. It's unclear why Niantic chooses not to announce the dinner raids.