When I was taking a dutch course, my teacher always told me it was "Vandaag is HET woensdag" ... I don't know if it's completely right without the "het", if someone could elaborate that would be helpful! (:
In everyday speech, I think you would more often hear 'vandaag is het woensdag': "today it is wednesday'. 'Vandaag is woensdag' is correct too though.
Het is woensdag, my dudes