Gregorian Calendar

From Turtledove

The Gregorian Calendar is a calendar introduced throughout Catholic Europe in 1582. It was designed by Calabrian doctor Aloyisius Lilius to correct a subtle flaw in the Julian Calendar which made the calendar slightly longer than the Earth's revolution around the Sun (though the Church continued to deny that the Earth revolved around the Sun) and decreed by Pope Gregory XIII. Protestant countries resisted the implementation of the calendar and continued to observe the Julian Calendar.

When Spain invaded and conquered England in 1588 and made Catholicism the official state religion of that kingdom, they imposed the Gregorian calendar by removing ten days from the month of June 1589.

In 1598, the Paschal Moon fell on the same day as the vernal equinox according to the Gregorian Calendar: a day which the Julian Calendar insisted was still winter, and thus not the Paschal Moon at all. Thus, Protestants and Catholics marked Easter on different days. In England, this forced Protestants who were concealing their identities for fear of Catholic persecutions either to violate the Catholic Lent, and thus reveal themselves; violate the Protestant Lent, and thus a tenet of their religion; or observe the grueling Lenten fast for over two straight months.