Extra Whitespace with ListBox and UniformGrid

Question

I'm making a calendar as an exercise, and I get the Listbox Items to generate correctly. However, I don't want any spaces in between the borders. I have set HorizontalContentAlignment to stretch, and that didn't fix it.

        <Grid>
            <ListBox x:Name="monthCalendarListbox"
                     Grid.RowSpan="2"
                     BorderThickness="0"
                     HorizontalContentAlignment="Stretch"
                     VerticalContentAlignment="Stretch">
                <ListBox.ItemsPanel>
                    <ItemsPanelTemplate>
                        <UniformGrid Rows="5"
                                     Columns="7"/>
                    </ItemsPanelTemplate>
                </ListBox.ItemsPanel>
                <ListBox.ItemTemplate>
                    <DataTemplate>
                        <Border BorderBrush="Black"
                                BorderThickness="1">
                            <StackPanel Background="Transparent"
                                        HorizontalAlignment="Stretch">
                                <Label Content="{Binding DayNumber}"/>
                                <Label Content="{Binding Message}"
                                       VerticalAlignment="Center"
                                       HorizontalAlignment="Center"/>
                            </StackPanel>
                        </Border>
                    </DataTemplate>
                </ListBox.ItemTemplate>
            </ListBox>
        </Grid>

EDIT:

It looks like ListBoxItem was getting a transparent border thrown in in addition to some padding (mentioned in the comments). So no more spacing but now I have double thick borders where the ListBoxItems meet. Any idea how to fix that?

        <Grid>
            <ListBox x:Name="monthCalendarListbox"
                     BorderThickness="0"
                     HorizontalContentAlignment="Stretch"
                     VerticalContentAlignment="Stretch">
                <ListBox.ItemContainerStyle>
                    <Style TargetType="ListBoxItem">
                        <Style.Resources>
                            <SolidColorBrush x:Key="x:Static SystemColors.HighlightBrushKey" Color="Transparent"/>
                            <SolidColorBrush x:Key="x:Static SystemColors.ControlBrushKey" Color="Transparent"/>
                            <SolidColorBrush x:Key="x:Static SystemColors.HighlightTextBrushKey" Color="Black"/>
                        </Style.Resources>
                        <Setter Property="BorderBrush" Value="Black"/>
                        <Setter Property="BorderThickness" Value="1"/>
                    </Style>
                </ListBox.ItemContainerStyle>
                <ListBox.ItemsPanel>
                    <ItemsPanelTemplate>
                        <UniformGrid Rows="5"
                                     Columns="7"/>
                    </ItemsPanelTemplate>
                </ListBox.ItemsPanel>
                <ListBox.ItemTemplate>
                    <DataTemplate>
                        <StackPanel Background="Transparent">
                            <Label Content="{Binding DayNumber}"/>
                            <Label Content="{Binding Message}"
                                   VerticalAlignment="Center"
                                   HorizontalAlignment="Center"/>
                        </StackPanel>
                    </DataTemplate>
                </ListBox.ItemTemplate>
            </ListBox>
        </Grid>