Kumbuka is the name of the annual celebration celebrating the anniversary of Simba's defeat of Scar.
The celebration is held on Pride Rock, on the anniversary of day that Simba put a stop to Scar's reign and took possession of the throne as was originally intended, subsequently returning peace to the Pride Lands. It celebrates this victory, hailing Simba as the true King of the Pride Lands.
Makini and the Lion Guard discuss Scar's defeat in The Lair of the Lion Guard. While Bunga is adamant that the victory lies with his uncle Timon and Pumbaa for doing a dance to distract the hyenas from the first movie, Makini and the rest of the Lion Guard remind him that Simba was the one who fought and defeated Scar. Kion further explains that this is what the Kumbuka is all about, though Bunga still believes his uncles were the real heroes.
At the Outlands Volcano, Scar is fully aware of the Kumbuka celebration, and hatches a plan with his army to take down Simba on this day, turning it into the day they remember as his return, and the day that he defeated Simba and The Lion Guard.
A short while later, the Kumbuka begins, with Rafiki presenting the event to the gathering of Pride Landers below. While he and the Pride Landers sing Good King Simba in the King's honor, the scorpionSumu quietly scuttles onto Pride Rock and injects his venom into Simba's tail. Simba collapses right after the song ends, and the Kumbuka celebration ends prematurely, with Pride Landers looking on in horror.