A year is about 365 days (except in a leap year). It is the time it takes the Earth to go completely around (orbit) the sun once. A year is actually almost 365.25 days long, but a calendar has 365 days, except in a leap year, which has 366 days.
The year starts on January 1 and ends on December 31 in the Gregorian calendar, but a fiscal year or a school year can start on a different day of the year.