I usually have lunch at two from Mondays to Fridays. 和 I usually have lunch at two from Monday to Friday (WITHOUT S) 的差別在哪裡？如果不好說明，請提供一些例句。

Adding the s usually means you're referring to all Mondays & Saturdays. Without the s it usually means you're referring to a specific Monday to Friday.

Example:
I will be in my office from Monday to Friday.
Implies you will only be there during those days, usually meaning the soonest Monday to Friday.

I am in my office Mondays to Fridays.
This implies you are in your office every Monday to Friday.

I hope I explained this well. It's a difference between whether you're referring to a specific time or not.