Year
From Wikislippy
A year is the time the orbit of the Earth around the Sun. By extension, this can be applied to any planet: for example, a "Martian year" is the time in which Mars completes its own orbit.
Calendar Year
A calendar year is the time between two dates with the same name in a calendar. Solar calendars usually attempt to predict the four seasons, but because the length of individual seasonal years varies significantly, they instead use an astronomical year as a surrogate. For example, the ancient Egyptians used the heliacal rising of Sirius to predict the flooding of the Nile.
The Gregorian calendar attempts to keep the vernal equinox on or close to March 21; hence it follows the vernal equinox year. The average length of a year is 365.25 days. In a non-leap year, there are 365 days, in a leap year there are 366 days. A leap year occurs every 4 years.
