I've always been a huge fan of twilight. There's something peaceful and calm about the death of a day. The world has rushed throughout the daylight hours to this point where it suddenly, collectively, takes a last breath and silence descends like a shroud.
I also enjoy the lighting at this time of day. It narrows your focus and you can highlight certain subjects with very little effort. Such as this grey mare juxtaposed against a dark backdrop, with her dark brown weanlings in the foreground.