Riot: When/How can we purchase or gift mystery chests?

For those who didn't watch MSI [SPOILER ALERT] there were Four Baron Steals in five games. I heard something about mystery chests being available with a % bonus chance (increased for each Baron steal!) for legendary skins.

I would like to know when these chests will be available. Will there be a notification or something on the client or message boards?

Thx if anyone knows the answer plz let me know.