Qu dia es hoy? What day is it today?

What you need to know
The days of the week
What order they go in

Make sure to remember
They days of the week en español don't start with Sunday, they start on Monday

To ask you say "¿Qué dia es hoy?". That means "What day is it?". To answer the question you have to say "Es..." than say the day. "Es..." means "It's..." in spanish. 

The Days of The Week	In Spanish and English
lunes	Monday
martes	Tuesday
miérocoles	Wednesday
jueves	Thursday
viernes	Friday
sábado	Saturday
domingo	Sunday

HERE is a video of when you would need this for a real life situation.
If you were in a spanish speaking country, and you wanted to know what day it was.