The premise is that weather is so ridiculously consistent within this campaign world that people can form days of the week out of it. (And without a day and night cycle, the planet being tidally-locked, the basis for a 'day' is far fuzzier anyway.) And because the notion of a week is largely born out of religious or astrological significance, we can incorporate that too.
Hopefully this solves two problems:
1) Not using enough weather effects in my games.
2) Providing opportunities for stealth and deception that nights typically offer.
Depending on which region one is in, the week has a different length. A "Day" in this case simply means somewhere approximately 20-30 of our hours in time, and each day describes the weather effects of that day.
So, for example. Sunday is a sunny day. Monday is a clear day in which the moon is visible. Rainsday is a rainy day. Giantsday is when the storm giants play bowling. Swordsday is when swords fall out of the sky. And so on and so forth.
If a random day is needed, roll the appropriate die.
Feyfjord / Twilight City / SouthlandsClimate: Cold Moderate
Inhabitants: Drow, Humans, Undead, Lycanthropes
Climate: Temperate Oceanic
Inhabitants: Humans, Orcs, Dragons
Roll 2d12, and take the higher result for the maximum number of days in this week (after which number it cycles back to one.)
Chronulus / Wizard City
Climate: Hot Dry
*This includes many of the mutant creatures which spawn and die during Sunday. Muuuutaaaant Ghooooooosts!