Rosh Hashanah is unique to Jewish holidays in that its history extends back to before we were a people. Pesach, Sukkot, Shavuot, and Yom Kippur all commemorate events that happened at or around the Exodus from Egypt.
Rosh Hashanah commemorates the creation of Adam.
It is certainly reasonable to believe that Rosh Hashanah was a holiday long before there were Jews to celebrate it.
The day was probably observed by the people who lived in the time of Adam. No doubt the tradition around this holiday made its way to Noach. And when Noach boarded the ark, in addition to the animals he also brought with him the traditions that he learned from Metushelach and the other elders who lived before the flood.
After the flood Noach's family continued to celebrate Rosh Hashanah and passed it down to their children. Eventually this tradition was passed down to Avraham who passed it onto his children who took the tradition down to Egypt. When the Torah was given, this day was included as one of the holidays.
The observance of Rosh Hashanah testifies that all men were created equal by God. Therefore the tyrants and despots of each generation sought to obscure this tradition as they usually predicated their power on a claim that they were divine and therefore superior to other men.
When Avraham celebrated Rosh Hashanah it was a direct affront to Nimrod who claimed to be a God in order to rule over others.
When our ancestors celebrated Rosh Hashanah under Egyptian slavery it was an act of defiance against Pharaoh who claimed to be God in order to enslave others.
When the Jews during the Holocaust celebrated Rosh Hashanah it was an act of defiance against the Nazis who claimed there is no God in order to exterminate a people.
In addition to being a day of prayer, the observance of Rosh Hashanah makes an ideological declaration that all men were created by God in the image of God and no man or group can claim innate superiority over another that gives them the right to rule, enslave or murder.
Rosh Hashanah also reminds us that we are all children of Adam and Chava and therefore all part of one family.
May the observance of Rosh Hashanah send a message of peace, brotherhood, and love to the world and may we all be inscribed in the book of life.