Since rarely does anyone use days of the week in its Nominative form:
1. Ponedeljak = Monday = Понедељак
2. Utorak = Tuesday = Уторак
3. Sreda = Wednesday = Среда
4. Četvrtak = Thursday = Четвртак
5. Petak = Friday = Петак
6. Subota = Saturday = Субота
7. Nedelja = Sunday = Недеља
Let me share with you a few exercises at three different levels:
B: What are you doing on + DAY ( Accusative case )? = Šta radiš u subotu?
Let's practice together:
C: Words "svaki / sledeći / naredni / drugi + day "will trigger the Genitive case of both the adjective and the noun in the question:"What are you doing next Monday?" = "Šta radiš sledećeg ponedeljka?"