In regard to:  "Calculating what day of the week Columbus reached on  Oktober 12, 1492 in America might be incorrect."  Columbus would have been using the "Julian Calendar", which reckons dates quite differently than using the "Julian Day Number", despite the similarity of the names.  Note that Julian Day number changes at Noon UTC rather than changing at Midnight local time where the Gregorian calendar and Julian Calendar assume the date changes.  October 12, 1492 (Julian Calendar) is October 21, 1492 (Gregorian), a 9 day difference, since there was a 10 day difference when the Gregorian calendar started on October 15, 1582 (October 5, 1582 Julian Calendar) and February, 1500 was a leap year in Julian Calendar but not in the Gregorian Calendar.  Both the Julian Calendar and the Gregorian Calendar (and the Islamic and Hebrew calendars) would agree that the first Columbus day was a Friday.  I.e., whenever any calendar adds a leap day/month or skips/adds days to switch between calendars, the days of the week just change by 1 normal day when we switch from one sunrise to the next.  Religions and countries may argue over what year of the calendar it is and what month of the year it is and what day of the month it is but there is much less argument over which day of the week it is.