“It is pleasant for an old man to be able to go to bed on September 2, and not have to get up until September 14,” wrote Benjamin Franklin in 1752. The reason for the shift was the adoption of the Gregorian calendar, devised in 1582.
Background
Roman Julius Caesar invented the modern calendar. Before then, different cultures and regions used their own calendars. These different calendars were oftentimes based on phases of the moon. This resulted in the same day falling in different parts of the year over time.
This problem still exists in some ancient calendars. For example, in the Muslim calendar shifts about 11 days earlier every year. Therefore, holidays cycle through different seasons over time. The Jewish calendar has a similar problem but added a leap-month to adjust.
As the Roman Empire expanded, they recognized the various calendars caused confusion for international panning. Rome needed for one consistent calendar where each day fell at the same time every year.
Roman astronomers computed if they divide the year into 365 days then add an extra day every fourth year the days will remain consistent. They named this the Julian Calendar, after Julius Caesar, and it remained the standard for almost 1,600 years.
Gregorian Calendar
Over time, Catholics realized the days were slowly shifting, noting that Easter was falling ever further from the spring equinox.
Catholic astronomers reworked the Roman computations. They realized the Julian Calendar added too many days, pushing days forward through the seasons. They invented a revised calendar, skipping leap years when the year is divisible by 100 unless the year is also divisible by 400. This new calendar resulted in a near-perfect match where the days of the year remain constant.
However, the Catholic Church wanted not only to start the new consistent calendar but also fix the date shift from the old calendar. They proposed rolling the calendar ahead 12 days then fixing it forever.
Catholic countries immediately adopted the change. However, Protestant countries imagined some type of Catholic conspiracy theory and resisted the change at first. Eventually, they relented. Germany switched in the year 1700 and England (including the then US colonies) in the year 1752.
The Greek and Russian Orthodox churches still have not switched. They celebrate Easter, though not Christmas, on a different day than the Gregorian Calendar.