I enjoy collecting holidays exclusively celebrated by fandoms and birthdays of fictional characters. Here's every holiday and birthday I could find for Pokemon.
Fandom Holidays:
Pikachu Day – February 2nd – Source
Pokémon Day – February 27th – Source
Satoshi Day – March 14th – Source
Kids Day – May 5th – Source
Pachirisu Day – August 8th – Source
Birthdays:
Moon – Pokémon Adventures manga – January 1st
Y – Pokémon Adventures manga – February 2nd
Mewtwo – Pokémon Red, Green, Blue, & Yellow – February 6th
Lorelei – Pokémon Adventures manga – March 15th
Diamond – Pokémon Adventures manga – April 4th
Crystal – Pokémon Adventures manga – April 30th
Blake – Pokémon Adventures manga – May 4th
Kellyn – Pokémon Ranger: The Comic – May 11th
Lunick – Pokémon Ranger the Comic – May 18th
Ash Ketchum – Pokémon anime – May 22nd
Green – Pokémon Adventures manga – June 1st
Ruby – Pokémon Adventures manga – July 2nd
Gold – Pokémon Adventures manga – July 21st
Giovanni – Pokémon Adventures manga – August 1st
Red – Pokémon Adventures manga – August 8th
Solana – Pokémon Ranger the Comic – September 16th
Whitley – Pokémon Adventures manga – September 16th
Sapphire – Pokémon Adventures manga – September 20th
White – Pokémon Adventures manga – October 5th
Black – Pokémon Adventures manga – October 8th
Platinum – Pokémon Adventures manga – October 27th
X – Pokémon X & Y – November 6th
Hayley – My Pokémon Ranch – November 18th
Blue – Pokémon Adventures manga – November 22nd
Kate – Pokémon Ranger: The Comic – December 1st
Silver – Pokémon Adventures manga – December 24th
Bill – Pokémon Adventures manga – December 31st
Most of these manga characters are counterparts to the main series game protagonists. They have different names, but share character designs. All of these birthdays are confirmed on Bulbapedia.
I am compiling Google Docs for all of the fictional/fandom holidays and birthdays I can find across media. If I am missing any Pokémon ones, please let me know! If it's been a while since this was posted, you can always DM me.
Further Reading:
In-Game Birthday Events – Bulbapedia
Festivals And Holidays Articles – Bulbapedia
by wintercool612