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 and Eastern Orthodox countries resisted the implementation of the calendar and continued to observe the Julian calendar for a time, though eventually they all adopted the Gregorian calendar, at least for secular date-keeping. The United Kingdom, for instance, adopted the Gregorian calendar throughout its empire in 1752. Russia adopted the calendar only after the October Revolution in 1917. The Russian Orthodox Church, like all Eastern Orthodox churches, continues to use the Julian calendar to determine the dates of its liturgical feasts.
Today the Gregorian calendar is the official civil calendar of nearly every country in the world, regardless of religion, and is the de facto international standard calendar. Other calendars continue to be used in many places to set the dates of various historical, cultural, and religious observances.
When Grantville arrived in the year 1631, the Gregorian calendar was primarily used in Catholic countries in Europe. Protestant and Orthodox areas and churches had largely refused to adopt it, as they saw it as Catholic, though a few Protestant rulers had adopted it.
However, in the world Grantville had left, the Gregorian calendar was the universal calendar. Thus, when Michael Stearns declared that there would be a celebration on July 4, 1631, certain of the town's allies briefly raised an eyebrow, but the issue did not cause any major conflict. It was apparently viewed as a local-option matter.