Humans have celebrated the start of each new year with parties, festivals, and religious ceremonies for millennia. However, we haven't always agreed on the beginning of the year. Four thousand years ago, the first new moon after the Vernal Equinox was regarded as the dividing line between the previous year and the new one in ancient Babylon. After Julius Caesar made major changes to the Roman calendar in 45 BCE, January 1 became the start of the new year for the first time. Ancient Romans commemorated the day with sacrifices to Janus, the Roman god of beginnings (after whom the month of January is named), as well as gift exchanges and large parties.