Every one of the calendar's thirty days carries the name of a divinity. Ahura Mazda himself opens the month, the six Amesha Spentas and a run of Yazatas fill out the rest, and the twelve months are named from the same roster, so twelve of the thirty day names recur as month names and a day can share its name with the month it falls in. When it does, the coincidence is kept as a festival, most famously Mihragan, when the day Mihr meets the month Mihr in the autumn. Structurally the year is plain: twelve months of thirty days each, closed by five epagomenal Gatha days named for the five hymns of Zoroaster himself, giving three hundred sixty five days with, in the ancient reckoning, no leap day at all. That omission is the reason three distinct versions of the calendar exist today rather than one.
The Shahanshahi, "Imperial," calendar has run uncorrected since late antiquity and has drifted roughly a month against the true solar year; the Qadimi, "Ancient," calendar differs from it by one further month because some communities inserted a correction in the medieval period that others did not; and the Fasli calendar, adopted by reformers in the twentieth century, adds a leap day on the Gregorian pattern specifically to keep Nowruz pinned to the spring equinox. The three are not variants of a settled question but three live positions, and which one a given community keeps is a marker of its own history.