The Quarter Day Left Over Every Year
One trip around the sun takes Earth a little more than 365 days. It takes about 365 and a quarter days. Our calendar, though, has only 365 days in most years. So every year the calendar falls about a quarter of a day behind the sun.
A quarter of a day does not seem like much. But think about what happens as years pass. After one year, the calendar is one quarter day behind. After two years, it is half a day behind. After three years, three quarters. After four years, the extra pieces add up to one whole day. 4 times one quarter equals 1.
To fix the problem, we add an extra day to the calendar once every four years. That day is February 29, and the year that has it is called a leap year. Most years that you can divide evenly by four are leap years. The rule has a few extra fine-tuning steps, but dividing by four is the main idea.
Without leap years, the calendar and the seasons would slowly drift apart. After a few hundred years, summer weather would show up in months we now call spring. People born on February 29 are sometimes called leap day babies. In most years they celebrate their birthday on February 28 or on March 1, and they get the real date only once every four years.