What is notable about the date Friday 15 October 1582 in Western history?

Question

Where was Expo 2010 held from 1 May to 31 October 2010, the most expensive Expo in the history of world's fairs with the largest site (5.28 square km), with more than 190 countries and over 50 international organisations participating?

Answer 1

It was the first day observed under the Gregorian calendar
The calendar was introduced as a modification of the Julian calendar, which observed a leap year every 4 years without exception unlike the new calendar which omitted a leap year if the year was divisible by 100, except for years divisible by 400. This reduced the average year from 365.25 days to 365.2425 days, and adjusted for the drift in the 'tropical' or 'solar' year that the inaccuracy had caused during previous centuries. The adjustment "erased" the dates of 10 days - the preceding Thursday was 4 October under the replaced Julian Calendar.