As a follow up to https://wisej.com/support/question/month-calendar-saturday-and-sunday-names-invisible-and-questions, this issue also applies to Clear-1, Clear-2 and Clear-3 themes.
Also, there is no FirstDayOfWeek property available. At design-time, the first day of the week is Sunday but at runtime, it shows Monday. The behaviour applies to all themes.
Will also add the FirstDayOfWeek property to override the localization.