April Fools' Day

April Fools' Day was the first April Fools' Day Event in Pokémon GO that took place between March and April in 2018, to celebrate real-world.

Features
The most recognizable feature of this event was changing regular sprites of Pokémon (static raster graphics of 3D models) into 8-bit sprites known from original Pokémon games.

Featured Pokémon
Shiny Murkrow were released at the start of the event:

These Pokémon had increased spawn rates in the wild and from Incense or Lure Modules:

News banners
Banners for in-game news changed during the event, portraying the Pokémon as 8-bit sprites. 

Pokémon 8-bit sprites
The sprites were available for the Pokémon in regions released by the time of the event: Kanto, Johto and Hoenn. Smeargle and Celebi from Johto, Nincada family, Spinda, Kecleon, Clamperl family, the Legendary Titans, Jirachi, and Deoxys from Hoenn had not been released by the time of the event, and their sprites are not available. Furthermore, shiny sprites and event sprites were not available.