No, it's not...

http://msdn.microsoft.com/en-us/library/system.datetime.dayofweek.aspx

Here is the umbraco implementation:

  /// <summary>
  /// Implements the following function
  ///   string date:day-name()
  /// </summary>
  /// <returns>The name of the current day</returns>
  public static string dayname(){
   return DateTime.Now.DayOfWeek.ToString();
  }

  /// <summary>
  /// Implements the following function
  ///   string date:day-name(string)
  /// </summary>
  /// <returns>The name of the day of the specified date or the empty string if the 
  /// date is invalid</returns>
  public static string dayname(string d){
   try{
    DateTime date = DateTime.Parse(d); 
    return date.DayOfWeek.ToString();
   }catch(FormatException){
    return ""; 
   }
  }