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January (/ˈdʒænjuːˌɛəri/ ^ⓘ JAN-yoo-AIR-ee) is the first month of the year in the Julian and Gregorian calendars and one of seven 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 (where it is the second month of winter) and the warmest month of the year within most of the Southern Hemisphere (where it is the second month of summer). In the Southern hemisphere, January is the seasonal equivalent of July in the Northern hemisphere and vice versa.

January starts on the same day of the week as October in common years, and starts on the same day of the week as April and July in leap years. January ends on the same day of the week as February and October in a common year, and ends on the same day of the week as July in a leap year. In all years, January begins and ends on the same day of the week as May of the previous year. January in common years immediately before other common years begins on the same day of the week as April and July of the following year and in leap years and years immediately before that, January begins on the same day of the week as September and December of the following year. In common years immediately before other common years, January finishes on the same day of the week as July of the following year while in leap years and years immediately before that, January finishes on the same day of the week as April and December of the following year.

January (in Latin, Ianuarius) is named after Janus, the god of beginnings and transitions; the name has its beginnings in Roman mythology, coming from the Latin word for door (ianua) since January is the door to the year.

Traditionally, the original Roman calendar consisted of 10 months totaling 304 days, winter being considered a month-less 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 (354 days). Although March was originally the first month in the old Roman Calendar, January became the first month of the calendar year under either Numa or the Decemvirs about 450 BC (Roman writers differ). In contrast, specific years pertaining to dates were identified by naming two consuls, who entered office on May 1 and March 15 until 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 seventh day after 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 Slovene, it is traditionally called prosinec. The name, associated with millet bread and the act of asking for something, was first written in 1466 in the Škofja Loka manuscript.^[1]

According to Theodor Mommsen (The History of Rome, volume 4, The Revolution, ISBN 1-4353-4597-5, page 4), 1 January became the first day of the year in 600 AUC of the Roman Calendar (153 BC), due to disasters in the Lusitanian War. A Lusitanian chief called Punicus invaded the Roman territory, defeated two Roman governors, and slew their troops. The Romans resolved to send a consul to Spain, and in order to accelerate the dispatch of aid, "they even made the new consuls enter on office two months and a half before the legal time" (15th of March).

• National Thank You month
• Weight Loss Awareness Month^[2] (United States)
• National Codependency Awareness Month^[3] (United States)
• National Mentoring Month (United States)
• Stalking Awareness Month [1] (United States)
• Slavery and Human Trafficking Prevention Month [2] (United States)

• Handsel Monday in Scotland and northern England – First Monday
• Coming of Age Day (成人の日, Seijin no hi) in Japan – Second Monday
• Third Sunday is Pongal in India and Feast of the Santo Niño in the Philippines
• Martin Luther King, Jr. Day in the United States – Third Monday
• Chinese New Year – between January 21 to February 20

• New Year's Day – January 1
• Independence Day in Haiti – January 1
• Three Wise Men Day, or Epiphany, in Latin America, Spain, Puerto Rico and Dominican Republic, and is, although not celebrated as widely or in the same way as in countries with a Spanish history, an official holiday in many European countries, for example Austria, Italy, Sweden, Finland, Liechtenstein, Slovakia and Croatia, as well as in parts of Germany and Switzerland.  – January 6
• Russian and Ukrainian Christmas Eve, also known as Svyat Vechir – January 6
• Plough Sunday in Scotland and northern England – Sunday after January 6
• Coptic and Russian Orthodox Church Christmas – January 7
• National Human Trafficking Awareness Day in the United States – January 11
• Makara Sankranthi (Festival of Harvest) in India – January 14
• The uniting of Moldavia and Wallachia under the same ruler in 1859, Romania – January 24
• Burns night in Scotland – January 25
• Republic Day in India – January 26
• Australia Day in Australia – January 26
• Auckland Anniversary in Auckland, New Zealand – Monday closest to January 29

• January's birthstone is the garnet which represents constancy.
• Its birth flower is the cottage pink Dianthus caryophyllus or galanthus.^[4]
• The Chinese floral emblem of January is the Prunus mume.^{[citation needed]}
• The Japanese floral emblem of January is the camellia (Camellia sinensis).^{[citation needed]}
• In Finnish, the month of tammikuu means the heart of the winter and because the name literally means Oak moon, it can be inferred that the oak tree is the heart of grand forest with many valuable trees as opposed to the typical Arctic forests which are typically pine and spruce. The photograph of a large tree covered with ice against a blue sky is a familiar scene during Finland's winter.
• The zodiac signs for the month of January are Capricorn (until January 19) and Aquarius (January 20 onwards).