Hola Shirley

You could add "pasado" to the translation; the reason why we didn't is because it is implicit, so if you say "el lunes" and it is clearly referring to the past (Hice...) you don't really need it, you understand it was last Monday. But you can also say:

Hice mis deberes tanto ayer como el lunes pasado. 

I guess we could have also omitted "last" from the English and say:

I did my homework both yesterday and Monday. 

You assume here it is referring to last Monday, by reading the whole sentence.

I hope it clarified it.

Saludos

Inma