Learn French: La boucherie est fermée le lundi. - The butcher's is closed on Mondays.

In French, using the definite article le before a day of the week indicates a recurring event. So, le lundi means "every Monday" or "on Mondays". If you were talking about a single, specific Monday, you would normally just say lundi without the article.

Why does fermée have an extra e at the end?

In this sentence, fermée acts as an adjective describing la boucherie. Because boucherie is a feminine noun, the word describing it must agree, so we add the feminine e to the past participle fermé.

Could I say le boucher est fermé instead?

No, because le boucher refers to the person (the butcher), while la boucherie refers to the shop. Saying le boucher est fermé would sound like the person himself is closed!