Am Sonntag koche ich

Info: If you want to say that something will happen on Saturday morning, you say "Samstagvormittag". That is also how it works with other times of the day, e.g. "Mittwochmorgen", "Donnerstagnachmittag".

If you would like to say that something will happen on a certain weekday, you would use the preposition "am".

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.