The solar year is 365 days long, plus about 6 hours. The calendar is not the same as the solar year, since the calendar is a scheme for combining days, weeks and months into a regular pattern to match a sequence of solar years.

Think of it this way: a period of 365 calendar days is actually shorter than a solar year. But some "calendar years" are 366 days long, and so are longer than a solar year. The combination of three regular calendar years ("common years") plus a leap year, comes very close to matching a sequence of four solar years.

Sosigenes and Julius Ceasar thought the match was close enough, when they instituted the Julian Calendar in 46 BC. By combining three common years with a leap year, they got a calendar year which is 365.25 days long, on average. But this means the Julian Calendar was in reality a 4-year cycle.

Centuries later it became evident that this 4-year cycle is actually slightly longer than four solar years. The Gregorian reform therefore attempted to match a much longer sequence of solar years by combining common years and leap years. Rather than add a leap-day every four years, the Greorian Calendar omits adding a leap-day in most century years (years ending in 00). An exception is made for century years divisible by 400; so the year 2000 will have a leap-day.

The Gregorian calendar therefore combines 303 common years with 97 leap years, in order to match a sequence of 400 solar years. The result is a calendar year 365.2425 days long, on average. Still, the match is not perfect.

But this means the Gregorian Calendar is actually 400 years long!