The evening sky goes through various stages of twilight as the sun journeys below the horizon. Knowing these stages can help you plan your shoot, especially for landscapes.
The period right after sunset, when the sky is colorful and is favorable for the typical landscape shot.
Dark enough to see the first star. Kinda dark for landscapes, but great for long exposure photography.
Most stars are visible. Pitch dark after this stage, when you can start astrophotography.
Note: These twilight stages happen in the morning as well, before sunrise, but in reverse order.