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.