Monday, Tuesday, Saturday, etc are 'days of the week'. Monday through Friday are 'weekdays' and Saturday and Sunday are the 'weekend'. Morning, afternoon, etc are 'times of (the) day' and each individually are a 'time of (the) day'. So, "What time of day is it there?" is answered with "It's evening."

I think I understand your question. (If I don't, tell me!) The word "Monday" refers to the 24 hour period that comes directly after the 24 hour period we call "Sunday." Don't let the "day" in those words fool you! This is why we say: Monday morning, Monday afternoon, Monday evening, and Monday night." If we talk about the daytime, without being exact about the time of day, we say, "Monday." Hope this helps. :-)

based on what I found, it's called "parts of the day"