ISO 8601 is not related to Unicode (ISO/IEC 10646), but if you do care about Unicode you may notice that Unicode CLDR actually supports the preferred first day

ISO 8601 is not related to Unicode (ISO/IEC 10646), but if you do care about Unicode you may notice that Unicode CLDR actually supports the preferred first day of the week as a part of locale identifiers [1] (e.g. en-US-u-fw-mon will make the first day of the week Monday, as opposed to the default Sunday for USA). [1] https://www.unicode.org/reports/tr35/#UnicodeFirstDayIdentifier https://www.unicode.org/reports/tr35/#UnicodeFirstDayIdentif...