I am still confused by above few comments. If explanation is Niedziela is subject, then it seems to me, as well, that it should read "Niedziela jest dzisiaj". These replies to original question confused me.
"Niedziela jest dzisiaj", as immery stated, would mean "Sunday is today" and be an answer to a "So... does anyone remember when's the next Sunday?" question. "Dzisiaj jest niedziela" answers "What day is today?" question.