Here you will find a list of the special events that have happened so far, so that you know what to potentially expect for future events :P
Halloween Events[]
This was the 1st Halloween Event:
Halloween Candy which gave temporary buffs were temporarily loot-able, here's what they looked like:
This was the 2nd Halloween Event:
Halloween themed Special Infected were temporarily added to the game which gave about 1,000 base exp per kill, which added with the event EXP Bonus and the player's EXP Bonus, most players were getting roughly 2,500 exp per kill.
Christmas Events[]
This was the 1st Christmas Event:
Christmas Candy which gave temporary buffs were temporarily loot-able, here's what they looked like:
The 2nd Christmas Event:
Christmas themed Special Infected were temporarily added to the game which gave about 1,000 base exp per kill, which added with the event EXP Bonus and the player's EXP Bonus, most players were getting roughly 2,400 exp per kill.
Easter Events[]
This was the 1st Easter Event:
The Bunny Mask was added as a Limited-Time cosmetic for the length of the event costing 200 Credits.
Easter themed Special Infected were temporarily added to the game which gave about 1,000 base experience per kill depending on the zone. These Infected also wore the Bunny Mask mentioned above.
Anniversary Events[]
On the 5th of September 2019, DF2 had it's 1st Anniversary which was celebrated with a special event:
Note: It was actually 50% Exp Gain and not 25% like the announcement shows.
On the 5th of September 2020, DF2's 2nd Anniversary happened, which included the event shown below:
Note: Lasted until the 9th.
Summer Event & Sale[]
The summer event was a special event that lasted from 07/31/2020 to 08/9/2020. The event details are shown in the discord image below:
July 4th Weekend Event[]
To celebrate July the 4th, there was a special +100% EXP gain event for the weekend.
Covid-19 Event[]
The Covid-19 event started on 03/25/2020 and lasted until 06/15/2020.