The control does not extend a way to customize the format of the header text because it displays the name of the month, whereas the format for the edit portion is applied to a specific date. By default, we obtain the names of the month from the current culture's DateTimeFormatInfo. The UltraCalendarInfo.DaysOfWeek collection provides a way to override the display names of each month (see LongDescription/ShortDescription properties of the DayOfWeek object).

The following code sample demonstrates how to solve the problem you describe here using the IUIElementCreationFilter interface:

using Infragistics.Win.UltraWinSchedule;
using Infragistics.Win.UltraWinSchedule.MonthViewMulti;

/// <summary>
/// Implements IUIElementCreationFilter for the purpose of customizing
/// the header text for the UltraMonthViewMulti control.
/// </summary>
public class MonthHeaderCreationFilter : IUIElementCreationFilter
{
    #region IUIElementCreationFilter Members

    void IUIElementCreationFilter.AfterCreateChildElements(UIElement parent)
    {
        MonthHeaderAreaUIElement headerElement = parent as MonthHeaderAreaUIElement;
        if ( headerElement != null )
        {
            TextUIElement textElement = headerElement.GetDescendant( typeof(TextUIElement) ) as TextUIElement;
            if ( textElement != null )
            {
                Month month = headerElement.GetContext( typeof(Month) ) as Month;
                if ( month != null )
                {
                    textElement.Text = string.Format( "{0}/{1}", month.MonthNumber, month.Year.YearNumber );
                }
            }
        }
    }

    bool IUIElementCreationFilter.BeforeCreateChildElements(UIElement parent)
    {
        return false;
    }

    #endregion
}