During holidays, the weather is always perfectly appropriate. For example, on the Fourth of July it's sunny (for barbecuing and fireworks) and on Christmas it always snows.
Related to Let There Be Snow, where someone asks for, and gets, Christmas snow in a usually non-snowy location. It may be a good idea to point out the fact that it may have never once rained on Halloween in fiction might be notable. Pick a Halloween Episode, any Halloween episode, and you'll realize quickly that it is always a nice night for trick-or-treating.
More to the point, it's never snowed on Halloween in fiction, even though there's a lot of parts of the world where it does so on occasion. Halloween thunderstorms aren't all that odd in the horror genre, though.
- Lampshaded and justified in Good Omens. The area where the child Antichrist lives always gets "appropriate" weather (snow on Christmas, Bonfire Night never gets rained out, etc.) because said Antichrist is unconsciously using his powers to warp reality and make it so.
- Harry Potter always has Christmas snow. Every. Single. Book.
- In a The 39 Clues Rapid Fire eBook, right when it is revealed that Grace is going to die, it begins to snow.
- In Phineas and Ferb it's always summer, so you have to imagine Independence Day is in there somewhere. And it's always sunny. (Except in the Christmas episode. Can you guess what the weather was then? Yup, snow. Again.) Anyone sensing a pattern? Also, they have a Christmas in July episode where they make fake snow.