When Alaska was purchased from the Russian Empire, the calendar was changed from Julian to Gregorian, which caused Alaska to “lose” 11 days completely.
An official ceremony took place at New Archangel (Sitka), Alaska. The agreed date of the Julian calendar was Saturday, 07 Oct 1867. The American’s recorded the date as being Friday, 18 Oct 1867 for the Gregorian calendar.
The days from Oct 8-17 do not exist in any official record pertaining to Alaska and now you know why. Alaska losing these day was not a unique event either.
This 11 day gap between the calendar has caused a ruffle in other countries when the switch was made. The Julian calendar was replaced with the Gregorian calendar in Britian in 1752, Turkey in 1917, Russia in 1918, and Greece in 1923.
It was common for “dual dating” to be used on documents during this transition for their record keeping to remain intact. If ever you see O.S. or N.S. on documents, they are abbreviations for Old Style (Julian) and New Style (Gregorian).
In 325 AD the Council of Nicea set the holiday of Easter (Resurrection Day of Jesus Christ) as being the first Sunday after the first Full Moon after the vernal equinox, a planetary, or solar, event where the Sun is exactly above the equator and gives an equal amount of day and night.
In 1582, Pope Gregory XIII established what became known as the Gregorian calendar for the purpose of stabilizing the holy day of Easter from moving out of alignment with the vernal equinox as determined 1,257 years earlier by the Council of Nicea.
Even though most of the Catholic countries in Europe would adopt use of the Gregorian calendar, it would take nearly four centuries for a majority of the world to use it as a measure of record keeping.
There are other calendar systems that still exist in the world, such as the Alexandrian, established by the Coptic Orthodox Church and also known as the Coptic calendar, used officially by Ethiopia (known to them as the Ethiopian calendar).
The British Empire used the Gregorian calendar which carried over during the Independence of the American colonies and the Russian Empire used the Julian calendar. This is why when Alaska was purchased, it “lost” 11 whole days in 1867.