The Torah does not particularly distinguish the day the world was created from the rest of the days of the year. Rosh Hashanah actually comemorates the sixth day of the creation, the day of man’s creation, for a celebration or holiday is not appropriate for the creation of the physical world, which was, after all, a further downward development in the chain-like evolution of the various spiritual worlds (seder hishtalshelut). The G-dly light was clothed in an extra concealing garment, and doubled and redoubled darkness prevailed, until one could erroneously believe that there are two authorities -- G-d and nature -- and even that the physical is the only reality.
The day of remembrance for the work of creation was established on the anniversary of the creation of man, bearer of the power to illuminate the physical world and elevate all the spiritual worlds. On this day the Jewish people accept G-d’s Kingship over the whole creation, as Adam did on the day he was created, and G-d grants them a good and sweet year.
(Igrot Kodesh, vol. 2, p. 172)
