In 1582, when the Gregorian calendar was adopted, 4 October was followed directly by 15 October.
Why it is true
The Julian calendar had drifted ten days behind the seasons. Pope Gregory XIII ordered them removed, and Portugal, Spain, Italy and Poland went from Thursday 4 to Friday 15 October. Other countries only switched centuries later.
Source: Encyclopaedia Britannica
Would you rather be asked first?
This is the reading side of the site, so the answer is in the sentence above — that is the whole point of it. Test yourself works the other way round: the question comes on its own, and the fact and the explanation only appear once you have answered.
