Question

Hoy es lunes. ¿Qué día es mañana?

Answer 1

Hoy es lunes. ¿Qué día es mañana? means Today is Monday. What day is tomorrow?
If you are wanting to reply in Spanish you would say Tommorow es el martes.

Answer 2

Actually you'd say "Mañana es Martes."

Answer 3

Martes

Answer 4

martes would be the answer since, that is the day after lunes... In Mexico the calendar starts out as lunes-->Monday martes-->Tuesday miercoles-->Wednesday jueves-->Thursday viernes-->Friday sabado-->Saturday Sunday-->domingo. Also in Spanish these are never capitalized.