Since You Asked: TGIFF (Thank God It's Fifth Friday)

I noticed that our extra day in this leap year comes on Friday, giving us five Fridays for the shortened month. I know leap year comes around only every four years, but how often do we get five Fridays in February?

— Connor O., Grants Pass

There haven't been five TGIFs in February since 1980, Connor, and there won't be five again until 2036, so live it up today.

Between 1904 and 2096 leap years that share the same day of the week occur every 28 years. But while you're leaping, remember that there are asterisks next to "every four years" that affect how often February gets five Fridays.

Jim Todd, an astronomer at Oregon Museum of Science and Industry in Portland, reminds us that the leap day was invented to keep the calendar in time with the astronomical year, which is 365.2422 days. But if we added a day every fourth year, eventually we'd get ahead of ourselves.

That's what happened with the Julian calendar, introduced by Julius Caesar in 45 B.C. It had a leap year every four years, and by the 1500s things had gotten way out of sync. Our Gregorian calendar, (named for Pope Gregory XIII, in whose reign it was introduced) was adopted in some European countries as early as 1582, but didn't gain acceptance in Great Britain and North America until 1752.

Astronomers have developed a set of elaborate rules for the leap day to keep the calendar in time with the solar system. Years divisible by four are leap years, but years that are divisible by 100 are not, unless the year can be divided by 400. This simple modification eliminates three leap years every few hundred years.

That means 1800, 1900 and 2100, for example, are not leap years, but 2000 and 2400 get an extra day. The year 2000 was notable in that it was the first time many countries had to use the 400-year exception.

This system brings the calendar and the solar year within about 30 seconds of each other. At this rate, it will take about 3,300 years for the calendar and the solar system to diverge by one day.

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