As well as being able to recite the days, it is also easy to say what day it is, by learning the Spanish word es 'is'.
But what if you would like to talk about when something is happening or has already happened? There is an easy way to do this in Spanish, by saying 'on'. If you want to say 'on', as in 'My friends arrived on Monday', you use the Spanish definite article el 'the'.
Mis amigos llegaron el lunes. This would be literally translated as 'My friends arrived the Monday'.