With their numbers dwindling, Midyat's Assyrian community rotates services throughout the old town's churches, so it's hard to know which one will be open. Your best bet is the Mor Barsaumo church, open most afternoons. It has a beautiful chapel with distinctive locally made artwork and lovely stonework.
Şen Cad. 21, southern end of the old city, Midyat, Mardin, Turkey