using "le" with days of the week

Question

On the quiz the question was: How would you say "I go shopping on Tuesdays" I selected Je fais les courses les mardis. It was marked incorrect with the correct answer being le mardi. Can you explain why Tuesdays wouldn't be les mardis?

Maarten Kamp · Accepted Answer

Kelly, 
It depends what you mean by ‘ explain ‘ it ? 
It is ‘ le mardi ‘ because that is the way habitual activities occurring regularly on certain days of the week are described in French. 
If you want to confirm  ‘ every Tuesday .. ‘, you can say ‘ tous les vendredis ‘, but this does have a different emphasis.
As per the lesson 
“ when meaning-
 "on Mondays", as a habit : Je vais au cinéma le lundi. I go to the cinema on Mondays. 
Le vendredi, je joue au tennis. On Fridays, I play tennis. 
On aime aller se balader le weekend. We like going for walks at the weekend.
Note that in these cases, the day remains singular in French: le vendredi “