Aquarium light question?

i have a pretty stupid question, but i don't know since im a beginner. are you supposed to leave the light on all day? or just during the day? or just during the night time? i have live plants and i need to know what to do so they don't die. if it matters i have 50 gallon tank only neon tetras and java moss and some other plants sorry i dont know the name. thank you :)