This custom System.EventArgs class provides additional information for the Calendar.VisibleMonthChanged event. The additional information consists of two properties: PreviousDate and NewDate (which will typically be the first of the newly selected month).

public sealed class MonthChangedEventArgs {
// Public Constructors
   public MonthChangedEventArgs(DateTime newDate, DateTime previousDate);
// Public Instance Properties
   public DateTime NewDate{get; }
   public DateTime PreviousDate{get; }
}

Passed To

MonthChangedEventHandler.{BeginInvoke( ), Invoke( )}