The earliest New Year's Day came about 50,000 BC. The ancient Egyptians had changed from nomadism to farming and settled on both sides of the Nile River. Their agricultural harvest was closely related to whether the Nile River flooded or not.
From long-term observation, the ancient Egyptians found that the Nile flooded regularly. They recorded the time on bamboo poles every time, and learned that the two floods were about 365 days apart.
They also found that when the rising tide of the Nile came near Cairo today, the sun and Sirius rose from the horizon at the same time. So the ancient Egyptians set this day as the beginning of the year. This is the earliest origin of New Year's Day.