Use el + [ day of the week] when an activity occurs on a specific day
Use los + [day of the week] when an activity occurs regularly
Except for sábados and domingos, the singular and plural forms for days of the week are the same
- El lunes tengo un examen.
- On Monday I have an exam.
- Los lunes y jueves tomo biologia.
- On Mondays and Thursdays I take biology.