The Julian calendar was the calendar officially replaced by the Gregorian calendar in 1582, contributing to different New Year dates across Europe.
Julius Caesar introduced the Julian calendar in 46 BC, and it began regular use in 45 BC. Its year length was slightly too long because the calendar assumed 365.25 days, while the tropical year is a little shorter. Over centuries, the discrepancy moved calendar dates away from the seasons.
Pope Gregory XIII introduced the Gregorian reform in 1582. It removed ten calendar dates to realign the calendar with the spring equinox and changed the leap-year rule: century years are not leap years unless divisible by 400. Catholic countries adopted the reform first, while many Protestant and Orthodox countries adopted it later.
Because adoption was not simultaneous, countries using different calendars could mark New Year's Day on different civil dates. Russia, for example, retained the Julian calendar for civil purposes until 1918. The “Old New Year” still survives in some communities using the Julian date.