xaml:

    <Window.DataContext>
            <local:MainViewModel/>
        </Window.DataContext>
        <Window.Resources>
            <local:MyCalendarConverter x:Key="DateConverter"/>
            <Style x:Key="CalendarDayStyle" TargetType="CalendarDayButton">
                <Style.Triggers>
                    <DataTrigger Value="True">
                        <DataTrigger.Binding>
                            <MultiBinding Converter="{StaticResource DateConverter}">
                                <Binding />
                                <Binding Path="DataContext.Dates" RelativeSource="{RelativeSource AncestorType=Calendar}" />
                            </MultiBinding>
                        </DataTrigger.Binding>
                        <Setter Property="Background" Value="Pink" />
                    </DataTrigger>
                </Style.Triggers>
            </Style>
        </Window.Resources>
        <Grid Margin="5">
            <Calendar CalendarDayButtonStyle="{StaticResource CalendarDayStyle}"/>
        </Grid>