Why was it dark today, when it hadn't been a few months ago? Well, I live in Illinois. Friday is the first day of Autumn. The sun now comes up later and sets earlier. Also, with rain in the offing, clouds obscured whatever light we might have had.
What does this have to do with writing? Well, you need to check the visibility ratio for the area and Season you're writing about before you make a flub and say it's dark when it's usually light, or vice versa. People who live in that location would definitely know the difference. You might also want to research whether or not that area is prone to draughts or heavy amounts of rainfall, since clouds can also diminish light.
If you don't have firsthand knowledge, you can ask a friend who lives there, or check the Internet.
Speaking of asking friends, How early does it get light around you these days?