Prepositions of time: an

Wann sieht Emma fern? –  Am Freitag.

When you say on which day you do something, you use the preposition an together with the article dem. The two words merge to form am.

When am is used to express time, it is often placed in first position in the sentence or immediately after the subject and the conjugated verb.

Am Freitag sieht Emma fern.

Emma sieht am Freitag fern.

Manchmal sieht Emma am Freitag fern.