[Editor’s note: Teo returns today for more calendar blogging. For more of his superb writing, check out his blogs: here and here.]

On this day in 1582, nothing happened in Spain, Portugal, Poland-Lithuania, or most of Italy. It’s not that this was an uneventful time in those places; far from it. This date, however, was right in the middle of the block of days eliminated from the calendar by the papal bull Inter gravissimas, issued a few months earlier, which recalibrated the civil calendar to bring the date of celebration of Easter back in line with where it had been at the time of the Council of Nicaea in AD 325 by declaring that the day after October 4 would be October 15. Since the bull was issued by Pope Gregory XIII,the resulting calendar is known as the Gregorian Calendar.

This is the calendar we still use today, of course, but it took a while for that to happen. The papal decree only took effect immediately in the parts of Italy where he was also the secular ruler, and the only other rulers to adopt the change on the intended date were Philip II of Spain and Portugal, Stefan Bathory of Poland-Lithuania, and the leaders of various small Italian states, all of them staunch Catholics. Other Catholic rulers, such as Henry III of France and the Austrian Habsburgs, adopted the new calendar within the next couple of years, while most Protestant countries resisted the change for more than a century. In the countries that did not accept the change originally, October 9 occurred as scheduled, and things happened on it. In Italy, Spain, Portugal, and Poland-Lithuania, however, October 19 occurred instead, and things happened then.