I have three celebrations based mostly on the artifacts and symbols of each holiday.
Christmas
(Tree Decorations)
1) Tree - Any pokemon resembling or growing tree/branches.
Exeggutor, Sudowoodo, Shiftry, Abomasnow, Torterra, Sawsbuck, Chestnaught, and Trevenant
2) Garland - Any Flower-Bearing Pokemon. Must learn Petal Dance or Petal Blizzard.
Venusaur, Bellosom, Jumpluff, Sunflora, Roserade, Cherrim, Maractus, Meganium, Vileplume, Florges, Lilligant
3) Tinsel - Any Serpetine-bodied Pokemon
https://bulbapedia.bulbagarden.net/...ody_style#Pok.C3.A9mon_with_serpentine_bodies
4) Lights - Any electric pokemon.
5) Bulb Ornaments - Any Pokémon consisting of only a head
Shellder, Cloyster, Gastly, Voltorb. Electrode, Koffing, Ditto, Sunkern, Misdreavus, Unown, Pineco, Forretress, Silcoon, Cascoon, Lunatone, Solrock, Castform, Shuppet, Glalie, Clamperl, Mismagius, Bronzor, Rotom, Whirlipede, Cottonee, Solosion, Duosion, Ferroseed, Cryogonal, Shelmet, Carbink, Klefki.
6) Angel Topper- Any fairy pokemon (Fairy type XY or fairy egg group other generations).
Kwanza
(The 7 Symbols)
1) Mazao (The Crops) - Pokemon that grow vegetation.
Exeggutor (coconuts), Sunflora (sunflower seeds) Amoongus (Mushroom), Parasect (Mushroom), Breloom (Mushroom), Tropius (Bananas), Torterra (seeds), Snover/Abomasnow, (snoverberries), Cherrim (Cherry), Gourgeist (Pumpkin), Chestnaught (Chestnuts).
2) Kinara (The Candles Holders) - Any flamed pokemon.
Charizard, Rapidash, Magcargo, Magmortar, Typhlosion, Infernape, Darmatian, Emboar, Pyroar, Chandelure.
3) Mkeka (The Mat) - Greninja (Mat Block)
Previous gens, any pokemon that learns reflect, barrier, or light screen by level-up.
4) Bendera (The flag) - Yanmega, Basculin, Flygon, Xatu, Cradily, Reuniclus, Siglyph, Tyranitar, Vivillion (Archipelago and Garden Patterns), Gogoat, Florges (Red), Hawlucha, and Trevenant. (Have a red, green, black component to color palette.)
5) Zawadi (The Gifts) - Gift Pokemon, gifted fossil, or in-game trade.
6) Kikombe cha Umoja - (The Unity Cup) - Pokemon must have the ability rain dish, sap sipper, or hydration.
Hanukkah
(Festival of Lights)
1) Menora - Any flamed pokemon.
Charizard, Rapidash, Magmortar, Magcargo, Typhlosion, Infernape, Darmatian, Emboar, Pyroar, and Chandelure.
2) Dreidel - Pokemon must learn Rapid Spin or Triple kick.
3) Star of David - Starmie, Crawdaunt, Solrock, Luxray, Volcarona, Geninja, Furfrou (Star Trim Only)
Must learn either swift, cosmic power, sunny day, flash, or water shuriken
4) Jelly-Filled Doughnuts - Exeggutor (eggs), Chansey/Blissey (eggs), Togetic/Togekiss (Eggs), Miltank (milk), Ludicolo (fruit), Tropius (fruit), Cherrim (fruit), Snover/Abomasnow (fruit), Vanilluxe (Cream), Slurpuff (Sugar), Gogoat (milk drink)
5) All work is forbidden - One pokemon must learn rest or slack-off by level-up/breeding.
6) Blue and White/Silver - Blastoise, Nidoqueen, Poliwrath, Lapras, Kingdra, Jumpluff, Azumarill, Gyarados, Dragonair, Vaporeon, Heracross, Swanna, Medite, Pelipper, Glalie, Altaria, Wailord, Walrein, Rampardos, Lucario, Samurott, Crygonal, Simipour, Jellicent (male), Beartic, Swoobat, Vanilluxe, Froslass, Pachirisu, Meowstic, Greninja, Aurorus,
Some of these examples are loosely based on concept. For example, Staryu/Starmie (barring legendaries such as Jirachi), are the only star pokemon. However, the other pokemon listed have a star design or are based on the sun (a star) or constellations. Allowing for the participant to be able to participate in a challenge using the game version of their choice. Otherwise, the challenge would be too restrictive and everyone would be using the same pokemon.
Also, I tried doing this quickly, so I didn't get all of the pre-evolution options up here.