La semaine = the week
Lundi = Monday
Mardi = Tuesday
Mercredi = Wednesday
Jeudi = Thursday
Vendredi = Friday
Le week end
Samedi = Saturday
Dimanche = Sunday
In French, we don't put capital letters for the days of the week and months. So, we write;
Je vais au parc dimanche = I go to the parc on Sunday.
Note that we don't say "on" Sunday but just "Sunday".
Also, if you say mardi, or jeudi, or samedi, that refers to a specific day. If you want to say every mardi or "on Tuesdays", say "the Tuesday".
Je vais a la gym le dimanche = I go to the gym on Sundays.
Le lundi = every Monday, on Mondays
Le week end = on week ends.
But say "pendant la semaine" = during the week.
Ecoutez ! Listen !