Faction Rallies are week-long events that are hosted once a month. They were first introduced in Destiny 2.
Faction Rallies are a competition between New Monarchy, Dead Orbit, and Future War Cult. Guardians can only pick one faction to side with. The Guardian's goal during the rally is to collect as many Faction Tokens as possible. Faction Tokens can be turned in for faction-themed weapons, armor, and shaders. Each Faction has a main reward, that will cost 1000 Glimmer for the faction's followers and 50,000 for non-followers.