In other words,

We have found that Thursday, October 4, 1582 (Julian) was followed by Friday, October 15, 1582 (Gregorian)

Both from the www.

A fellow I knew in university explained to me the troubles he had digging into papers from the time of the Fr Rev due to the date change. He was doing his master's and he said it was not only difficult to work with but also a SOB to footnote his thesis.