6m- Randomly opened, in random directions. I think this happens more in the day, but I'm not a 6m expert. Strictly speaking this is VHF, not HF, but it's often included on HF rigs.
10m- This is more consistently opened, especially in times of higher solar activity. Evening tends to be the best time, at least that's what I've found, but this is a daytime band.
12-17m- These are traditional daytime bands, usually opened. The longer the band in this window, the more reliable it is, and the more likely it is opened night or day.
20m- Can be opened day or night. This is the most reliable band for dx.
40m- Daytime is opened in a "local" area, give or take 500 km or more with a good set up. Night time opened worldwide.
80m- Similar to 40, but even more extreme.
160m- This one is only opened at night, and tends to be a more local area. Not unlike 6 meters, strictly speaking this is MF rather than HF, but it's often included on HF rigs.