"I work on Monday."
Translation:Yo trabajo el lunes.
Because when something happens on a specific day or days, you use the article el or "los when it's a repetitive action, e.g. On Saturdays he goes to the movies = Los sábados él va al cine. Here's a good explanation about it: https://www.thoughtco.com/how-to-use-definite-articles-3079100.