"Tomorrow is Thursday."
Translation:Mañana es jueves.
Why in this sentence is el jueves not used and in some sentences el is used before the day of the week?
Because the definitive article el when used with days of the week translates to 'on'. When a day of the week follows the verb Ser, it's not required.
hoy es martes - today is Tuesday
salimos el lunes - we leave on Monday