Answer: Rosh Hashanah is the day of the spiritual birth of Adam. Rosh Hashanah has been celebrated for 5,777 years, since the time when a person with the name Adam discovered the spiritual world for the first time.
That is how the possibility for discovering the spiritual world appeared among people and the beginning of the ascent toward it by leaving the state of the beast, leaving our physical world.
This is talking about a unique point of spiritual birth in which the first Adam (Man) went out to the spiritual world and that is how he paved the way for us. All the rest of the Kabbalists followed his way and developed his method based upon rising above egoism by attracting the Upper Light. This is the only method, and we are following the same way; so we mark this date.
From the Kabbalah Lesson in Russian 1/1/17