Q. When do the months commence, in Nisan or Tishri?
A. Though the Torah tells us that Nisan is “the beginning of the months” (Ex. 12:2), Rosh HaShanah is in Tishri and there are some who list the months from Tishri and not from Nisan, six months earlier.
There is a second aspect to the question, which is why the Biblical names for the months have not been maintained. Originally Nisan was Aviv, the month of spring. Other Biblical names include Ziv, the month of flowers (I Kings 6:1), Etanim, the month of perennial streams or the month of mighty ones (I Kings 8:2) and Bul, the rain month (I Kings 8:38). These names appear to have been displaced during the Babylonian period but we cannot be certain as to when.
The appointment of Nisan as the first of the months must have been widely accepted as an internal Jewish calendrical arrangement (II Samuel 11:1 calls Nisan “the return of the year”; in Jer. 36:22 the king “was sitting in his winter palace in the 9th month and the fire was burning before him”, which makes sense only if the 9th month is cold and is counted from Nisan in the spring).
Why then is Rosh HaShanah in the 7th month? Because, as explained in the Mishnah Rosh HaShanah, there are several new years, each with its own flavour and meaning. Rosh HaShanah, according to some, is at the beginning of the northern hemisphere autumn, when people’s thoughts turn to serious concerns and resolutions and repentance are appropriate.