why do we celebrate Rosh Hashanah in Tishrei?
After blowing the shofar on Rosh Hashanah we recite ‘Today is the birthday of the world…’. This would indicate that the world was created in Tishrei, and not in Nissan. This 6-month difference is due to a Talmudic debate as to whether one counts from God’s conceiving of the Creation (Tishrei), or His actual act of Creation (Nissan).