Why is there a February 29 every four years, and who came up with ths idea?
February 29th is called an intercalary or leap day. It is the extra day added during leap years, which, instead of the usuall 365 days, have 366. There is actually a very plausible reason why leap years and leap days were "invented," other than giving headaches to people born on February 29th.
An extra day is added every how many years to keep the calendar working properly. Our calendar is meant to match the time it takes for the earth to revolve completely around the sun--or the solar year. However, the 365 days in an ordinary calendar year is in fact a bit shorter than the actual time it takes for the sun to go around the earth, which is 365 and about 1/4 of a day ( 365 days, 5 hours, 48 minutes, adn 46 seconds to be exact). To catch up with the solar year, an extra day is added every four years to the calendar year.
Some sources say the Egyptians were the first to come up with the idea of adding an extra day to the calendar once every four years, but many say that it was Julius Caesar who actually became the first to designate February 29th as the leap day, and that leap years occur once every four years.
Everything seemed fine after Caesar, until the late 1500s, when people noted that the seasons were coming in earlier than usual. It turns out that the solar year is actually 11 minutes and 14 seconds LESS than 365 and 1/4 days long, so designating an extra day every 4 years may be a good idea, but it isn't accurate. The 11 minutes and 14 seconds of lag time between the solar & calendar years piled up since 45 BC, and by 1582, the calendar year was found to be AHEAD of the solar year by about 11 days, with the vernal equinox falling on March 11th instead of the 21st.
To correct this, Pope Gregory XIII adjusted the calendar dates and instituted an exception to Caesar's leap year rule: that century years be counted as leap years only if divisible by 400. This rule eliminates 3 leap years every few hundred years. Now, instead of the solar year lagging behind the calendar year by 11 minutes & 14 seconds, there is only about a half-a-minute difference.
Which isn't much. At least for the next 3,300 years.