QCalendarWidget automatically assumes that the "weekend" is always on Saturdays and Sundays, but this is not always correct and, most importantly, it's also inconsistent with the result of QLocale::weekdays().

For example, based on the most recent QLocale private data, all matching locales for India show that the weekdays also include saturday, and many Middle East countries report the fifth day of week ("friday") as a non-weekday, and the seventh ("sunday") as a weekday instead.

Doing such an arbitrary assumption is inappropriate. I suggest to improve it in order to always follow the current locale, possibly adding the possibility to explicitly override weekend days similarly to what's done with the firstDayOfWeek property.