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In the Julian calendar, edicted by Julius Caesar in 46 B.C. and still in use by the Orthodox Church today for the determination of religious holidays, every fourth year was a leap year. This was obtained by redoubling the 24th February, which in Roman parlance was "the sixth day before the Calends of March", hence the name "bissextile" (from bis-sextus, twice-the-sixth) used in many languages to name a "leap" year.
In the Gregorian calendar, edicted by Pope Gregorius XIII in 1582 A.D., every fourth year is a leap year except that secular years (i.e. "round centuries") are only leap when the year is a multiple of 400 (IOW, 1600 and 2000 were leap, but 1700, 1800 and 1900 weren't). The Gregorian calendar became legal immediately in Spain and Portugal, and progressively thereafter at various dates in most other countries of the world.
Another effect of Pope Gregorius XIII's reform was that 10 days were deleted in October of 1582 to bring the calendar back into agreement with the seasons.