When to use le in front of days of the week?

Bonjour Nicole,

You use the day of the week without an article if you are talking about a one-time event. 

Are you going to the library Sunday?  means only this Sunday.

    =   Tu vas à la bibliothèque dimanche?

For something that is repeated each week on a certain day, use the article before the day. 

My grand-daughter goes to chemistry lab on Thursday. (every Thursday.) = 

Ma petite-fille va au laboratoire de chimie le jeudi.

Thank you I understand now 

to make the distinction in English, we make the day of the week into a plural (by adding -S): ie., - on Thursdays. This is commonly used to express 'every Thursday'.  "He goes to the gym on Thursdays" - means he goes regularly on that day.