10 Tishrei 5711 = 21 September 1950
Yom Kippur is not a commemoration of an event. It is a day of fasting, reflection, and prayer for forgiveness for the sins of the past year. The Torah set the date for Yom Kippur on the tenth of Tishrei in the Jewish calendar. According to the Gregorian calendar, Yom Kippur falls in September or October.
Not this year. Earth Day is the same date in the Gregorian calendar each year, and the Gregorian calendar is not synchronized with the lunar calendar at all.
In the Gregorian calendar, December 31st
31 May 1949.
19 September 1979
The Julian calendar was used in most places that use the Gregorian calendar today, especially in countries considered "Christian". The switch to the Gregorian calendar, which is very similar but 25 times more accurate, began on October 15, 1582 (Gregorian date).
Some places were still switching from the Julian Calendar during the 20th century, but the first group of countries to switch to the Gregorian Calendar did so on October 15, 1582.
The date not only changes each year it changes daily. That is part of the purpose of a calendar.
April 10, 1955 Western, using the Gregorian Calendar April 17, 1955 Eastern Orthodox, using the Gregorian Calendar April 4, 1955 Eastern Orthodox, using the Julian Calendar Note: In the Gregorian Calendar the dates differ by seven days (Sunday to Sunday) The Julian and Gregorian Calendars have different dates. Most dates are reckoned using the Gregorian calendar now.
The Tanabata Festival is held on July 7th on the Gregorian calendar.
The Gregorian calendar is a solar calender, that was first used in 1582. This is where the start of leap year came into play. This is also how they began to determine the date of Easter.
The Catholic Church, i.e. Pope Gregory, instituted the Gregorian Calendar. However, it is now the standard calendar that is recognised by nearly everyone, regardless of religion. I.e., today's date in the Gregorian Calendar is 12 July 2012.