Learn Spanish (Latin America): La oficina está abierta desde el lunes hasta el viernes. - The office is open from Monday to Friday.

Both are correct and mean basically the same thing! Using desde (since/from) and hasta (until) just puts a little more emphasis on the specific starting and ending points of that time duration, similar to saying "from Monday all the way through Friday."

Why do we say el lunes and el viernes instead of just lunes and viernes?

In Spanish, days of the week almost always require the definite article (el for singular, los for plural). When you use the prepositions desde and hasta, you must include the article. However, if you use the shorter phrase de lunes a viernes, the articles are usually dropped.

Why is the verb está used here instead of es?

We use está (from the verb estar) because being open is a temporary physical state or condition. The office's status changes when it opens and closes. As a quick refresher: we use ser (like es) for permanent traits or identity, but estar for conditions that can change.

Why does abierta end with an -a?