which day of the week

[macalwen]

October 4, 1582 week 4, before this date the Julian calendar was used, with a leap year every 4 years, and working backwards from this, then January 1, 1 AD was week 6, October 5 to 14, 1582 was canceled, so October 15, 1582 was week 5, and from this date onwards the Gregorian calendar was used, with a slightly different calculation of leap years. To take all of this into account, it can only be calculated separately.