Often when a TV series has a yearly schedule, filming happens two to six months before the actual air date. However, because of the airing schedule, the episode is set on the date it is aired, and not when it was filmed. In Southern California locations, it ends up with perpetual spring, except when the production crew realize it's time for the Christmas Special Episode. But in other, more temperate climates, like Hollywood North, or pretty much anywhere else, it results in mismatched seasons.
On any given episode that is not specifically time framed, the trope bothers virtually no one. However, when you realise it is supposed to be, say, Valentine's Day, but trees still have falling leaves, or, say, Summer Holiday, but the trees don't have their leaves on yet, and a scenery shot is displaying full-blown daffodils (bloom season: March to April), the illusion is broken. Especially annoying when one single detail, like a date on a newspaper, is enough to ruin the suspension of disbelief.
This trope is different from Weather Dissonance, as the issue is almost never aknowledged on screen, except when lapshaded for comical effect, and from Bizarre Seasons because, although delayed, the seasonal pattern is still predictable.