The Sensational Six event originally came out on October 1st, 2020. This event has changed since it originally came out, and now lasts for the standard 11 days. You will need Mickey and Friends or Duck characters in order to unlock Daisy Duck, Goofy, and Donald Duck. You will then need to farm Minnie Mouse and Pluto in order to have all five characters needed for Sensational Six - Part 2.
In Sensational Six - Part 2, you will use Daisy Duck, Goofy, Donald Duck, Minnie Mouse, and Pluto to unlock Mickey Mouse.
As a bonus, the Sensational Six - Part 2 event is the first legendary event to have a "Prestige" tier which will unlock additional gear for you to use to level your characters. You will need the Sensational Six leveled to high gear levels in order to play the "Prestige" tier.