Gregory merely modified the Julian calendar, refining the calculation the leap day that by 1582 had resulted in 10 extra days. So, in October 1582, the 4th was followed by the 15th.
In a leap-year, February is given a 29th day. A leap-year is any year divisible 4, as designed by Julius Caesar. However, Gregory modified this, in that all double-zero years (divisible by four) would only be leap-years if also divisible by 400.
Within this, it uses weeks of 7 named days, but these do not directly correspond to specific dates, so vary month to month and year to year. These are of Western tradition, but are probably not directly linked to the Gregorian calendar. I've provided the English names because this document is in English, but many other languages have similar or drastically different names for a seven-day-week:
I've cross-referenced this calendar in my perpetual calendar with several other solar calendars.
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