This is great! I hope it is like this for other holidays and/or months
Here are some ideas:
Thanksgiving: Um...Pidgey line or swellow line
Winter Holidays: Sneasel, Delibird, Jynx(but I have other plans for it!), Swinub line, Sheal line
Valentines Day: Jynx
Spiring Holidays: Oddish line, grass starter lines, almost all other grass pokemon
Beginning of Summer: Lapras, a lot of other water pokemon including water starter lines
End/Middle of Summer: Magmar, fire and electirc pokemon
My Birthday: Chikorita, Treecko, Lapras, Amphoros, Forretress, Lugia
Halloween: Like we have now, ghost pokemon, but also dark, psycic and stell all fit halloween
Dragon, Normal and Legendary pokemon would be somewhat random(but not totaly)
Treecko