The time for the Earth to complete one orbit around the Sun is a Sidereal Year, and is equal to roughly 365.25 Mean Solar Days.
Due to the precession of the Earth, the time is takes to go from one Vernal Equinox to the next Vernal Equinox, a Tropical Year, is about 20 minutes shorter than a Sidereal Year.
The Vernal Equinox is the moment that the Sun (on the Ecliptic, its fictitious orbit around the Earth) crosses the Celestial Equator going northward. The Vernal Equinox is the moment that the season of Spring begins.
This 20 minute discrepancy between the Sidereal Year and the Tropical Year caused a serious problem with European calendars, which did not get corrected until the introduction of the Gregorian Calendar.