The official Pokémon Twitter account has recently confirmed distribution information for March's legendary event. You can find out more about the two Pokémon in the below Tweet!
Pokémon fans in the US and Canada will need to be signed up to the Pokémon Trainer Club newsletter by March 1st to get a special code for either Heatran or Regigigas. Players with Pokémon Sun and Pokémon Ultra Sun will be able to get Regigigas, while owners of Pokémon Moon and Pokémon Ultra Moon will be able to get a Heatran.
If you're in Europe then the event will be held over the Nintendo Network, so you'll just need to use the mystery gift option in-game. This event will begin running from the 1st of March until the 24th. Last months event for Dialga and Palkia is still live until the 28th of February and more information for it can be found here.