How to change the size of some elements of WPF DatePicker

I'm writing a C# WPF application.
In a window i put two DatePicker objects: I was able to enlarge the font of the calendar and enlarge the image icon.

I also need to enlarge the font of the two first lines of the calendar (month + year) and the row of the days of week (see the image ..).

I created a copy of the style for DatePicker control but I do not identify the element to change in it. I defined a new setter for CalendarDayButton, CalendarDayButton, CalendarItem to set font to 36px but I dont see the result.

How can achieve that, please?

Let me show you a way using the VisualTreeHelper.

First, you need to have a way to search controls through the VisualTree.

public static IEnumerable<T> FindChildren<T>(DependencyObject parent)
    where T : DependencyObject
{
    for (int i = 0; i < VisualTreeHelper.GetChildrenCount(parent); i++)
    {
        DependencyObject child = VisualTreeHelper.GetChild(parent, i);

        if (child is T childAsT)
        {
            yield return childAsT;
        }

        foreach (T grandChild in FindChildren<T>(child))
        {
            yield return grandChild;
        }
    }
}

Then, in the DatePicker's CalendarOpened event handler, we can find the Popup and controls inside it. The Popup.Child is a Calender control, which has 50 TextBlocks inside it. The items with index 1 to 7 are the TextBlocks for the days of the week.

private void DatePickerControl_CalendarOpened(object sender, RoutedEventArgs e)
{
    if (FindChildren<Popup>(DatePickerControl).FirstOrDefault() is not Popup popup)
    {
        return;
    }

    double fontSize = 20;

    if (FindChildren<Button>(popup.Child)
        .Where(x => x.Name == "PART_HeaderButton")
        .FirstOrDefault() is Button headerButton)
    {
        headerButton.FontSize = fontSize;
    }

    if (FindChildren<TextBlock>(popup.Child) is IEnumerable<TextBlock> textBlocks)
    {
        int targetChildIndex = 1;

        for (int i = 0; i < 7; i++)
        {
            TextBlock dayOfTheWeedTextBlock = textBlocks.ElementAt(targetChildIndex);
            dayOfTheWeedTextBlock.FontSize = fontSize;
            var text = dayOfTheWeedTextBlock.Text;
            targetChildIndex++;
        }
    }

    if (FindChildren<Grid>(popup.Child)
        .Where(x => x.Name == "PART_YearView")
        .FirstOrDefault() is Grid yearViewGrid)
    {
        foreach (CalendarButton calendarButton in FindChildren<CalendarButton>(yearViewGrid))
        {
            calendarButton.FontSize = fontSize;
        }
    }

    if (FindChildren<Button>(popup.Child)
        .Where(x => x.Name == "PART_NextButton")
        .FirstOrDefault() is Button nextButton)
    {
        nextButton.RenderTransform = new ScaleTransform(1.5, 1.5);
        nextButton.RenderTransformOrigin = new Point(0.5, 0.5);
    }

}