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January is the first month of the year in the Julian and Gregorian calendars, and one of seven Gregorian months with the length of 31 days. The first day of the month is known as New Year's Day. It is, on average, the coldest month of the year within most of the Northern Hemisphere and the warmest month of the year within most of the Southern Hemisphere.

January is named for Janus (Ianuarius), the god of the doorway; the name has its beginnings in Roman mythology, coming from the Latin word for door (ianua) - January is the door to the year. Traditionally, the original Roman calendar consisted of 10 months, totalling 304 days, winter being considered a monthless period. Around 713 BC, the semi-mythical successor of Romulus, King Numa Pompilius, is supposed to have added the months of January and February, allowing the calendar to equal a standard lunar year (355 days). Although March was originally the first month in the old Roman Calendar, January became the first month of the calendar year either under Numa or under the Decemvirs about 450 BC (Roman writers differ). In contrast, years in dates were identified by naming two consuls, who entered office on May 1 and March 15 before 153 BC when they began to enter office on January 1.

Various Christian feast dates were used for the New Year in Europe during the Middle Ages, including March 25 and December 25. However, medieval calendars were still displayed in the Roman fashion of twelve columns from January to December. Beginning in the 16th century, European countries began officially making January 1 the start of the New Year once again — sometimes called Circumcision Style because this was the date of the Feast of the Circumcision, being the 8th day from December 25.

Historical names for January include its original Roman designation, Ianuarius, the Saxon term Wulf-monath (meaning wolf month) and Charlemagne's designation Wintarmanoth (winter / cold month). In Finnish, the month is called tammikuu, meaning month of the oak, but the original meaning was the month of the heart of winter, as tammi has initially meant axis or core. This month is in Czech called leden, meaning ice month.

• New Year's Day – January 1
• Handsel Monday in Scotland and northern England – First Monday
• Coptic Christmas – January 7
• Plough Sunday in Scotland and northern England – Sunday after January 6
• Coming-of-Age Day (成人の日 Seijin no hi) in Japan – Second Monday
• Martin Luther King, Jr. Day in the United States – Third Monday
• Presidential Inauguration in the United States - January 20 at Noon EST
• Australia Day in Australia – January 26
• Republic Day in India – January 26
• Makara Sankranthi (Festival of Harvest) in India – January 15
• Auckland Anniversary in Auckland, New Zealand – Monday closest to January 29

• January's birthstone is the garnet.
• Its birth flower is the carnation or snowdrop.^[1]
• The Chinese floral emblem of January is the plum blossom.^{[citation needed]}
• The Japanese floral emblem of January is the camellia.^{[citation needed]}