Radio frequencies above ~9000 kHz work best during daylight hours, and those below ~12000 kHz work best when it's dark (at night).
MW (AM): ~525-1700 kHz. "Practical groundwave reception typically extends to 200–300 miles, with greater distances over terrain with higher ground conductivity, and greatest distances over salt water. Most broadcast stations use groundwave to cover their listening area." (Wikipedia) US stations are limited to 50 kW with a useful daytime radius of about 100 miles. MW list by frequency and region