For example, on the Fourth of July, the sky is always going to be sunny and clear (for barbecuing and fireworks), and on Christmas, snow is essentially inevitable. This makes sense considering those events happen in the middle of American summer and winter respectively, but also seemingly ignores all the various, sometimes random factors that affect weather. Just because it snows in a certain day in a certain year doesn't mean that next year will be the same after all. This can lead to geographical inaccuracies if a specific location in the US starts exhibiting weather that very rarely or never actually materializes in that region for the sake of holiday atmosphere.
This mostly happens with Christmas, but Halloween is also a notable offender. Pick a Halloween Episode, any Halloween episode, and you'll realize quickly that it's always a nice night for trick-or-treating, despite the abundance of rainfall during the season. At best, you get a cloudy, windy night for some spooky atmosphere, but thunderstorms only show up if it's an actual horror story with a killer or monster set loose. Valentines' Day is another common case of this. Technically, it's a winter holiday and spring is over a month away, yet Valentine's Day Episodes will always be spring-flavored.
See Empathic Environment for weather that changes to reflect character actions. Related to Let There Be Snow, where someone asks for, and gets, Christmas snow in a usually non-snowy location. Dreaming of a White Christmas is a Sub-Trope.
- 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. Most likely, A Wizard Did It.
- 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.
- Rocko's Modern Life: Played with in the Christmas Episode, where O-Town hasn't had a white Christmas in years and it just rains instead. It miraculously starts snowing when Rocko and his new elf friend manage to bring Christmas cheer back to O-Town, but by the day after Christmas the snow has already started to melt.