technique - What information about a fictional world is unnecessary?

I was able to gain some insight already thanks to How much detail is too much?, but I still need a more precise answer, because my details aren't bound to a particular scene.

I was searching for some information on tidally locked planets (my setting for the story) and while looking at the questions over at World Building SE I noticed that people had chosen specifical temperatures, days per year, etc, whereas I never even thought about building the world so precisely. I did think about the key elements of such a place (wind, no day-night, temperature), but I didn't choose a year cicle being 33 days long or something.

My story is a fantasy, with magic and weird creatures, so it doesn't have to be completely scientific, but I still want a world that is believable. I thought it didn't really add anything of value to the story if a place is -25°C or -30°C, it should be clear that it's pretty cold, that's it.

Am I mistaking? Is it better to explain a setting meticulously or stick to the main elements to make the setting more believable?

Answer

You have made a common mistake about world-building: believing that it all has to go on the page. World-building is for you, the author, to help you craft a story in a setting that feels real and unique, even though fictional. The actual details that make it to the page are only what the characters and reader need to know.

Knowing details like the actual length of the year or exact average temperature might help you avoid committing jarring inconsistencies, or writing descriptions that are so vague they feel insubstantial. So it's important work that helps the story eventually. But you absolutely don't need to shoehorn it into the narrative.

Some writers, like Murakami, or Diana Wynne Jones, get away with making up their worlds as they go along, at the price of a certain insubstantial, dreamlike quality to their settings. Of course, even if you're J.K. Rowling, who is known as the archetypal "planner," you can still end up leaving the occasional detail feeling wrong, inconsistent or poorly thought out. But in general, world-building helps your fantasy world feel more substantial, even (or especially!) if you keep the details to yourself.