January is the first month of the year in the Julian calendar and Gregorian calendars and the first of seven months to have a 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 April and July in leap years. It ends on the same day of the week as February and October in common years and July in leap years. In common years preceding leap years or leap years preceding common years, it begins on the same day of the week as September and December of the following year and ends on the same day of the week as April and December of the following year. In common years preceding common years, January begins on the same day of the week as April and July of the following year and ends on the same day of the week as July of the following year. January also begins and ends on the same day of the week as May of the previous year.
Ancient Roman observances during this month include Cervula and Juvenalia, celebrated January 1, as well as one of three Agonalia, celebrated January 9, and Carmentalia, celebrated January 11. These dates do not correspond to the modern Gregorian calendar.
