Blue hour

Blue hour sits either side of golden hour, on the far side from the sun. The sun is below the horizon. The sky is lit indirectly and the light reaching you is diffuse, cool and free of hard shadows. It rarely lasts an hour, more like twenty to forty minutes depending on latitude and season, and it changes fast. City work suits it best, since street lights and windows read warm against a deep blue sky while there is still enough ambient light to hold shadow detail. Bring a tripod, shoot faster than you want to, and set white balance by hand, because auto will try to correct the blue away.