According to the newest video posted by The Official Pokémon YouTube Channel, various Legendary Pokémon distributions will be held this year to "Celebrate the year of Legendary Pokémon".
Here is a list of the Legendaries being distributed and what month they'll be held:
February: The box Legendaries of DP (Dialga and Palkia)
March: Regigigas and Heatran
April: Entei and Raikou
May: The box Legendaries of XY (Xerneas and Yveltal)
June: Zygarde (Shiny)
July: Thundurus and Tornadus
August: The box Legendaries of RS/ORAS (Kyogre and Groudon)
September: The [email protected] (Latios and Latias)
October: The box Legendaries of BW/B2W2 (Reshiram and Zekrom)
November: The box Legendaries of HS/HGSS (Lugia and Ho-oh)
Upon further and more detailed research, you can get the one that is not respective to your game, except Zygarde.
I.E.: From the distribution event, you can get Palkia if you have Sun/Ultra Sun and Dialga if you have Moon/Ultra Moon. This would still require you to go through the Ultra Wormhole "Warp Drive" to get the one that IS respective to your game.
With these new distribution events, they'll let you encounter the other Legendaries in USUM's Ultra Wormhole "Warp Drive" sidequest with ease, especially those which require certain other Legendaries to be present in your party to encounter them.
Those certain others being:
Rayquaza = Kyogre + Groudon
Landorus = Thundurus + Tornadus
Suicune = the remaining two Legendary Dogs
Kyurem = Reshiram + Zekrom
Giratina = Dialga + Palkia