If the words يوم and نهار have one meaning, it means that one of them is useless, nowadays, we use the word يوم in an astronomical sense, wich means 24 hours (and it's legal) . on the other hand; نهار means the period between sunrise and sunset. If I said "سألتقي بك يوم الجمعة" ", it means at any hour on Friday, daylight or night. but if I sadi " سألتقي بك نهار الجمعة" it means only in daylight. Even though, you can use يوم in the place of نهار, but the opposite is incorrect. it's not correct that يوم and نهار have the same meaning, and it's tricky, so don't confuse yourself.

يوم means all 24-hours. نهار means the part of the day that start with sunrise and ends with sunset.