<DataGrid AutoGenerateColumns="False" ItemsSource="{Binding TestList}">
<DataGrid.Columns>
<DataGridTemplateColumn Header="Start">
<DataGridTemplateColumn.CellTemplate>
<DataTemplate>
<TextBlock Text="{Binding Start, StringFormat=d}" FontFamily="Verdana" />
</DataTemplate>
</DataGridTemplateColumn.CellTemplate>
<DataGridTemplateColumn.CellEditingTemplate>
<DataTemplate>
<DatePicker SelectedDate="{Binding Start}" FontFamily="Verdana" >
<DatePicker.CalendarStyle>
<Style TargetType="Calendar">
<Setter Property="DisplayMode" Value="Month"/>
</Style>
</DatePicker.CalendarStyle>
</DatePicker>
</DataTemplate>
</DataGridTemplateColumn.CellEditingTemplate>
</DataGridTemplateColumn>
</DataGrid.Columns>
</DataGrid>