Am Montag arbeite ich im Büro.

On Monday I work in the office.

Breakdown of Am Montag arbeite ich im Büro.

• ich: I
• arbeiten: to work
• im: in the; (masculine or neuter, dative)
• das Büro: the office

Questions & Answers about Am Montag arbeite ich im Büro.

Why is it am Montag and not just Montag for “on Monday”?

In German, days of the week normally take a preposition when you say “on [day]”.
The usual phrase is:

• am Montag = literally “at/on the Monday” → idiomatic English: “on Monday”

am is a contraction of an dem:

• an (preposition, “at/on”) + dem (dative masculine article “the”)
  → an dem Montag → am Montag

Because Montag is masculine (der Montag), and an with time expressions uses the dative case, you get an dem Montag, which is always contracted to am Montag in normal speech and writing.

Just saying Montag arbeite ich im Büro is possible, but it sounds more like a heading, note, or very informal style (e.g. in a planner). The full, normal sentence would use am.

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What is the difference between am Montag and montags?