According to Serebii.net, the largest Pokemon fan site, To celebrate Pokemon’s 22nd Anniversary, a special version of Pikachu will be available in the game from February 28th at 21:00 UTC.
SUMMARY OF THE EVENT – Party Pikachu!
Event Period: From February 28th at 21:00 UTC
Special Rewards: “Partychu” with a special move Present and will give triple Stardust on catch
Both Party Pikachu and Raichu can be shiny: