Translation:Yo no trabajo los lunes.
I think because that's like saying, "On mondays, I don't work." The word order is different
Coz you need to have articules before name of the day. El lunes- monday. Los lunes- mondays
"On Mondays" is expressed using "los lunes" in Spanish - you don't add 'en'
Yo no trajabo los lunes doesn't male sense. Isn't that " I do not work on THE Mondays"?
Interesting to note that the plural of lunes is lunes... whereas the plural of sabado is sabados, domingo is domingos... i.e. this sentence for sunday would be yo no trabajo los domingos. My first quess at this was yo no trabajo los luneses, thinking I had to make lunes plural... but not so.