Where did the day lunes come from in spanish?

Answer 1

Being a so-called 'Romance' language, i.e. one derived from Latin (along with French, Portuguese, Italian, Romanian, and to some extent, English), because of its country being occupied for many years by the Romans, Spanish takes the basic idea from the Latin word for the Goddess of the Moon - 'Luna'.
Just as 'Moon-day' has changed into 'Monday' in English, so in Romance languages
'luna+dies' (Latin moon + day) has changed into:
'lunedi' - Italian
'lundi' - French
and
'lunes' - Spanish.
The regional apocopation, or shortening, might be compared to the way Standard English has variations in British regions, American English (cf 'hi' or 'howdy' for 'hello' and 'how do you do'), Australian English ('g'day' for 'good day'), etc.

Answer 2

I don't know the official reason, but I've heard that it's a religious concept. The Lord rested on the seventh day, which would be Sunday (domingo) and so that's why it's last. On a more practical note, it means weekends are together on the calendar.