Here's why:
* They are all nouns: Lundi, mardi, mercredi, jeudi, vendredi, samedi, dimanche.
* French nouns that end in "-di" are typically masculine: This is a common pattern in French grammar, although there are exceptions.
Therefore, you would use the masculine definite article "le" before each day of the week:
* Le lundi
* Le mardi
* Le mercredi
* Le jeudi
* Le vendredi
* Le samedi
* Le dimanche