Buy the 2015 Old Farmer's Almanac!
The tropical yearThe tropical year (essentially, the cycle of seasons, or how long it takes the Earth to orbit around the Sun) is about 365.2422 days long (rounded up to nearest ten thousandth). The Gregorian calendar is usually 365 days long. To compensate for the "0.2422" day (slightly under a quarter of a day each year) to keep in tune with the seasons, the calendar adds a "leap" day every four years, except when century years are not divisible by 400. With this rule, 97 leap days are added every 400 years, which means the average length of year in the Gregorian calendar is 365.2425 days. The skipping of certain centuries helps to keep the calendar more in step with the tropical year. Although 365.2425 is still a bit ahead of the tropical year, it won't be out of step a full day until about 3300 years. If we didn't have this extra "divisible by 400" rule, the average year in the Gregorian calendar would be 365.25 days instead of 365.2425, which means the calendar would even more quickly become out of step by a day with the tropical year (in about 128 years).

2015 Special Edition Garden GuideCooking Fresh with The Old Farmer's AlmanacThe Almanac Monthly Digital MagazineWhat the heck is a Garden Hod?