Hi Ivan,
There are actually several brushes that come into play with MonthCalendar items. You are setting the "normal" brush, but not the "disabled", "hover", or "pressed" brushes. Since you are setting the Foreground with a Style, it has the same priority as the default/implicit Style. The default/implicit Style (which provides the themes) will change the Foreground based on the current state.
I'm assuming you currently have SelectionMode set to Single, in which case the day-of-week items are actually disabled. When you set SelectionMode to Extended and set IsDayOfWeekSelectionEnabled to true, then you would see the red foreground.
What you can do instead is change the brush resources used by the default/implicit Style. Something like this:
<Style x:Key="MonthCalendarWeekItemStyle" TargetType="{x:Type editors:DayOfWeekItem}"
BasedOn="{StaticResource {x:Type editors:DayOfWeekItem}}">
<Style.Resources>
<SolidColorBrush x:Key="{x:Static themes:EditorsCommonDictionary.CalendarForegroundNormalBrushKey}" Color="Red" />
<SolidColorBrush x:Key="{x:Static themes:EditorsCommonDictionary.CalendarForegroundDisabledBrushKey}" Color="Red" />
</Style.Resources>
</Style>
This creates a Style that is based on the default/implicit Style, but alters the normal and disabled brushes. You could also change the hover and pressed brushes. Finally, I'm assuming you've hooked up the Style to the MonthCalendar like so:
<editors:MonthCalendar DayOfWeekItemStyle="{StaticResource MonthCalendarWeekItemStyle}" ... />
