私はこれのためにテンプレートを変更する必要かもしれないと思います。内部の境界線のCalendarItemスタイルの一部です。
<Style x:Key="CalendarItemStyle1" TargetType="{x:Type CalendarItem}">
<Setter Property="Margin" Value="0"/>
<Setter Property="Padding" Value="0"/>
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type CalendarItem}">
<ControlTemplate.Resources>
<DataTemplate x:Key="{x:Static CalendarItem.DayTitleTemplateResourceKey}">
<TextBlock Foreground="#FF333333" FontWeight="Bold" FontSize="9.5" FontFamily="Verdana" HorizontalAlignment="Center" Margin="0,6,0,6" Text="{Binding}" VerticalAlignment="Center"/>
</DataTemplate>
</ControlTemplate.Resources>
<Grid x:Name="PART_Root">
<Grid.Resources>
<SolidColorBrush x:Key="DisabledColor" Color="#A5FFFFFF"/>
</Grid.Resources>
<VisualStateManager.VisualStateGroups>
<VisualStateGroup x:Name="CommonStates">
<VisualState x:Name="Normal"/>
<VisualState x:Name="Disabled">
<Storyboard>
<DoubleAnimation Duration="0" To="1" Storyboard.TargetProperty="Opacity" Storyboard.TargetName="PART_DisabledVisual"/>
</Storyboard>
</VisualState>
</VisualStateGroup>
</VisualStateManager.VisualStateGroups>
<Border BorderBrush="{TemplateBinding BorderBrush}" BorderThickness="{TemplateBinding BorderThickness}" Background="{TemplateBinding Background}" CornerRadius="1">
// get rid of the border thickness here and it's gone
<Border BorderBrush="Purple" BorderThickness="2" CornerRadius="1">
<Grid>
<Grid.Resources>
<ControlTemplate x:Key="PreviousButtonTemplate" TargetType="{x:Type Button}">
カレンダーのパディングではありませんか? –