Based on our current callendars, a year lasts 365.25 days.
This means that it actually takes the earth 1/4 more of a day to reach the same exact position around the sun than it was at a certain point the prior year.
But, because our calendar is not able to show this additional 1/4th of a day (or 6 hours), we tend to skip it. We end up missing out on 6 hours every year.
If you think about it, this means that each year, we are technically celebrating New Year's 6 hours earlier than the last, but I don't want to delve into this right now.
To compensate for this missing 1/4th of a year, we add an additional day every four years, that way our calendars will be on track again. That way, Christmas will be celebrated when the Earth is approximately at the same point around the sun as it was last year, four years ago, even ten years ago.
To think of it differently, if we didn't add that extra day at all, after 100 years, we would be celebrating Christmas on December 1st, even though the calendar would say the 25th. By my 'December 1st' in that last sentence, I am talking about the point around the sun we are normally positioned on that day. In 100 years, we would lose 25 days. Adding a day for every four years keeps us in place.
Or does it?
The Ancients (Mayans, Aztecs, and even other "primitive" cultures) based their calendars on a year of 365.24 days, not 365.25 like we do now. Even today's scientists claim that these ancient calendars are more accurate than our modern ones. So, what's with a difference of 1/100th?
Well, the easiest thing to do is simply multipy the numbers by 100. Fortunately, 100 is also the number of years that makes up a century, so let's keep that in mind.
With our modern calendar, a century will contain 36,525 days (365.25 times 100). The more accurate ancient calendar shows that a century only has 36,524 days. So, every 100 years, we're adding one too many days!
After 100 years, we're celebrating New Year's on January 2nd!
Considering that the majority of the world uses the Christian Calendar (I think even the Chinese recognize our calendar, although they still use their own), I think we need to re-evaluate our calendar system. I think that the best system would be to keep our leap years coming every four years, except that every century, we skip one. The best time to skip would be the double 0 years... 2000 (we've already passed this one), 2100, 2200, etc.
The biggest problem, of course, is getting the rest of the world to see it my way.