It’s not so much a formal event as it is an easter egg to remember our good friend, Kevin Kanai Griffith, for whom Kanai’s Cube is named.
The blog Blackarrows references goes into more detail about this hidden treasure, except when the cosmetic chest opens up (the last week of March - it commemorates Kevin’s real-life birthday). It was meant to be a hidden bonus rather than something we advertise or outline.
It is always a bit of a bittersweet memory, but we hope you take some time to adventure alongside Kanai and remember the incredible work he did bringing Diablo III and Reaper of Souls to life.
Miss you, KKG.