“…This month shall be to you the head of the months; to you it shall be the first of the months of the year…” – Shemos 12:2
הַחֹדֶשׁ הַזֶּה לָכֶם רֹאשׁ חֳדָשִׁים רִאשׁוֹן הוּא לָכֶם לְחָדְשֵׁי הַשָּׁנָה – שמות יב, ב
The Jewish calendar year has two starting points. Rosh Hashana, on the 1st of Tishrei, marks the start of a new year since the creation of the world and mankind. However, we are commanded to reckon the month of Nissan as the first of the twelve months of the year, in commemoration of our miraculous redemption from Egypt in this month. Hence, Nissan too is the head of the year.
The start of the year in Tishrei is only logical. For in addition to marking the years since creation, the calendar year that begins with Tishrei also coincides with the natural processes of the agricultural cycle. Tishrei is the season for planting, and we harvest the fruits of our labor in the spring and summer that follow.
Moreover, upon careful observation, we can discover that even the processes that seem entirely natural are actually orchestrated from Above, with a precision that is nothing short of miraculous. We therefore reckon the months from Nissan, to emphasize that the produce we harvest in the spring and summer is not the natural result of our efforts six months earlier, but of the Divine blessing that transcends nature entirely.
—Igros Kodesh, vol. 28, pp. 166-168