Depending on which game you have. (Latios- Alpha Sapphire / Latios - Omega Ruby)
26th November 2014 -
This event is still ongoing and most likely will be as it says here that there is no end date.
To get these event Pokémon you will need an Eon Ticket.
> It cannot be obtained normally in-game; instead, it must be downloaded at a promotional event or using the Eon Ticket e-Card
The information on the Eon Ticket and how people get it are in the source.
The Unova Starters Event
Can be obtained in either Omega Ruby or Alpha Sapphire.
The codes for these were all released at different times but the event end on the 30th November 2015.
To get these you will need to enter three seperate codes.
Serperior's is: POKEMON497
Samurott's is: POKEMON503
and finally Emboar's is: POKEMON500
They said that you needed to get an email from Pokémon Trainer Club but I got mine without.
Other events were listed in the source above (but they are now finished) and I'm guessing you don't want Japanese events?
If you do then ask and I'll put them on. Hope I helped :)
EDIT: in case you didn't know the Unova starters are events as they each have their Hidden Abilities.