To ask someone what day is today in spanish you say "¿Qué dia es hoy?"
If you want to say "it's" in spanish you say "Es",  before you say a day you say "Es jueves (Thursday)"
"Today" in Spanish is "hoy"
Day in Spanish is "dias"

Don't get mixed up with American ways of writing days of the week. *NOTE* - the days of the week en espanol are not capitalized.

lunes - Monday

martes - Tuesday,

miércole - Wednesday

jueves - Thursday

viernes - Friday

sábado - Saturday 

domingo - Sunday

When someone asked you, "¿Qué día es hoy?" (which means: What day is today?) you reply, "Es viernes" [It is Friday] 

For the calendar when someone ask you, "¿Qué día de la semana es el 5 de Octubre? you respond, "el martes"

Semana = week.

Real life Application: When someone is asking what is today or the days of the week.