A Colossal Discovery is a currently unannounced Pokemon Go event revealed by Pokemon Go dataminer Chrales, and you need a ticket to get in. The $8/£7.99 ticket can be seen on the app store page for Pokemon Go under In-App Purchases, although nothing official has been released about the actual event. Given the name of the event, it's all-but certain that the "Colossal Pokemon" Regigigas will be the event's big catch.
0.157.0 new item "Ticket: A Colossal Discovery"A ticket to access the "A Colossal Discovery" special event on November 2 from 11:00 AM to 7:00 PM local time, wherever you are. pic.twitter.com/3G2Tp1wnkiOctober 1, 2019
This is sort of a big deal, and let me explain why. Niantic has previously organized ticketed events, like Pokemon Go Fest, but the ticket grants access to a specific location rather than a brand new legendary. There have also been global events, but they've been available to everyone, with or without a ticket.
A Colossal Discovery is raising eyebrows because it's a global event that charges an entry fee and rewards buyers with a new legendary Pokemon that currently can't be found anywhere else, and there doesn't seem to be a way to access the event without paying real money. The hefty ticket price only makes matters worse. If it ends up selling, we're likely to see more of these types of events in the future, which might not sit well with everyone in the community.
If you don't care about a potentially dubious microtransaction and just want to make sure you catch a Regigigas, the Colossal Discovery event runs from 11AM to 7PM local time on Saturday, November 2.