[The gamemode is not available anymore]
The 2Years Anniversary Event is an event was been added in update 0.10.11 and the goal is collect as much event exp as possible to complete the event pass. Through this pass you can get 2Years Collection skins and 2Years Medals.
The conversion of normal exp to event exp is roughly:
1 event exp = 3 normal exp (this is a rough estimate, the exact conversion is unknown)
For more information on exp and score points please visit Level System.