In 1582, the Catholic Church decided to fine-tune the calendar they used since they felt Easter didn’t fall anymore on what they thought was the correct day every year. And thus the Gregorian Calendar was invented to replace the Julian Calendar. When that was done, they had to get rid of 10 days to re-adjust the yearly cycle. So for the Church, the last day of the Julian calendar was Thursday October 4 1582, which was followed the next morning by the first day of the Gregorian calendar, Friday October 15 1582.
But not everyone was Catholic, or followed these Catholic decisions in the matter of calendaring. Catholic countries switched from the Julian to the Gregorian system relatively quickly (Spain, Italy, Poland, etc), but others followed much later (Russia, 1917; Greece, 1923). The United Kingdom and its colonies (like Canada and the American Colonies) did the switchover in 1752 and got rid of 10 days when switching calendar: Wednesday September 2 1752 was followed by Thursday September 14 1752 for these people.
This can be seen in Ubuntu. I’m running on this laptop in Canada using the en_CA.UTF-8 locale. And when I run the “cal” command from the command-line, I get:

So we see correctly (at least for me in English Canada) that the missing 10 days in 1752, but not in 1582. I haven’t checked but I’m assuming (and hoping) that in other locales the missing 10 days would appear in the correct year for that particular location. You can get a long list of these actual Julian to Gregorian switchover dates for various countries and regions at this web site.
We also have a bug report currently open (and confirmed) in Launchpad about not removing 10 days in 1582 (Bug #20276). When I first commented in that bug report last year, I was under the assumption that indeed we needed to remove 10 days in the output of the cal command for 1582, and it seems the output was still wrong for the year 1752 back then (obviously that has been solved since). Now, after reading a bit more this morning about all this, I’m starting to think that maybe this bug should be rejected since that missing 10 days has to appear on a country-by-country basis, and not in 1582 for everyone.