wrzesień

Wrzesień is September, named from wrzos, heather. In dates it takes the genitive form września: dnia ósmego września, on the eighth of September. Without diacritics it is often typed wrzesnia in indexes.

Unlike English September, the Polish name has nothing to do with the number seven, so do not expect a link to Latin month numbers. Do not mix up września with sierpnia, August, the month before. The Roman numeral for September is IX.