Musar על בראשית 2:9

In Midrash Rabbah (Bereishis 30:8) it is taught that every individual to whom the term tamim — “flawless” or tamimus — “completion” applies, completed a unit of seven “weeks,” corresponding to the forty-nine days. Three days prior to the giving of the Torah Israel was commanded in the mitzvah of “separation” [hagballah], at which time they purified themselves like a woman purifying herself for her husband after menstruation. On the fiftieth day Israel merited to enter the last of the fifty Gates of Understanding, which is called the “Tree of Life that is in the midst of the garden” (Bereishis 2:9). Although there is a dispute between Rabbi Eliezer and Rabbi Yehoshua (Rosh HaShanah 11a) regarding whether the world was created in Nissan or Tishrei, both agree that it was not actualized until Nissan. Therefore it is from the first of this month that we number the months. Thus Nissan is called the “first [lit. head] of the months.”