Hello! Preposition on + day of the week.
Present Simple (actions which you do regularly)

I play tennis once a day. (Present Simple)
I play tennis on Fridays. (Present Simple)
I play tennis on Monday. (This sentence is not correct)

But, if I say
I sometimes play tennis on Monday. (Present Simple)
I usually go shopping on Saturday. (Present Simple) Are these sentences correct?

My question is Can I say in the Present Simple?
I sometimes play tennis on MONDAY.
I usually go shopping on SATURDAY.

Should I add (s) at the end of Monday and Saturday?

I sometimes play tennis on MondayS.
I usually go shopping on SaturdayS.

Question

Hello!  Preposition on + day of the week.
Present Simple (actions which you do regularly)

I play tennis once a day. (Present Simple)
I play tennis on Fridays. (Present Simple)
I play tennis on Monday. (This sentence is not correct)

But, if I say  
I sometimes play tennis on Monday. (Present Simple)
I usually go shopping on Saturday.   (Present Simple) Are these sentences correct?

My question is Can I say in the Present Simple?  
 I sometimes play tennis on MONDAY. 
I usually go shopping on SATURDAY.

Should I add (s) at the end of Monday and Saturday?

I sometimes play tennis on MondayS. 
I usually go shopping on SaturdayS.

Answer

I play tennis on Monday. (This sentence is correct)
I sometimes play tennis on Monday. (Present Simple)
I usually go shopping on Saturday. (Present Simple) Are these sentences correct? - Yes, they are correct.
Both variants are possible.
I have Spanish lessons on Tuesday. = I have lessons on Tuesdays.

Ne manquez pas cette occasion d'apprendre une langue depuis chez vous. Découvrez notre sélection de professeurs de langues expérimentés et inscrivez-vous dès maintenant à votre premier cours !