In Gems of War a new Weekly Event starts in a different Kingdom each week after Weekly Reset.
Select the Events icon from the World Map for information on the News - the new Troop and the Guild Event for that week - and the Events tab - for information on the Weekly Event and the types of Troops that have bonuses that are available by using certain types of Troops.
The Events Tab
Each week two groups of Troops receive stat bonuses. This may include all Troops from the event Kingdom or a Troop of a specific type (i.e. Orcs) belonging to that Kingdom. These bonuses are related to the current Guild Event that is running, for example, the Invasion Event may use Orcs and Troops from Grosh'Nak, so both Troops with the Orc type will be available as well as any Troop from Grosh-Nak.
Earn Kingdom Troops with Event Chests
The best time to earn specific Troops is when the Weekly Event for that Troop's Kingdom comes. Throughout the week only Troops from the Weekly Event Kingdom will be available in Event Chests.