using this

DataTable dt = new DataTable();
        dt = GetGeneric.GetInstance<AdminDriverHelper>().GetDriversHolidays("");

        dt.Columns.Add("Id", Type.GetType("System.Int64"));
        dt.Columns.Add("EventStartDate", Type.GetType("System.DateTime"));
        dt.Columns.Add("EventEndDate", Type.GetType("System.DateTime"));
        dt.Columns.Add("EventHeader", Type.GetType("System.String"));

        // int idCount = 1;
        DataRow dr;

        for (int i = dt.Rows.Count - 1; i >= 0; i--)
        {
            DataRow row = dt.Rows[i];
            dr = dt.NewRow();
            dr["Id"] = Convert.ToInt64(dt.Rows[i]["DriverID"]==DBNull.Value?"0":dt.Rows[i]["DriverID"]);
            dr["EventStartDate"] = Convert.ToDateTime(dt.Rows[i]["DriverDate"]==DBNull.Value?DateTime.Now:dt.Rows[i]["DriverDate"]);
            dr["EventEndDate"] = Convert.ToDateTime(dt.Rows[i]["DriverDate"]==DBNull.Value?DateTime.Now:dt.Rows[i]["DriverDate"]);
            dr["EventHeader"] = Convert.ToString(dt.Rows[i]["DriverName"]==DBNull.Value?"":dt.Rows[i]["DriverName"]);
             dt.Rows.Add(dr);
        }

        return dt;

I still get this

Object cannot be cast from DBNull to other types.

Description: 
An unhandled exception occurred during the execution of the current web 
request. Please review the stack trace for more information about the error and 
where it originated in the code. 

Exception Details: 
System.InvalidCastException: Object cannot be cast from DBNull to other 
types.

Source Error:

An unhandled exception was generated during the execution of the 
current web request. Information regarding the origin and location of the 
exception can be identified using the exception stack trace below.

Stack Trace:

[InvalidCastException: Object cannot be cast from DBNull to other types.]
   System.DBNull.System.IConvertible.ToDateTime(IFormatProvider provider) +56
   System.Convert.ToDateTime(Object value) +32
   ExtendedControls.EventCalendar.EventCalendarDayRender(Object sender, DayRenderEventArgs e) +321
   System.Web.UI.WebControls.Calendar.OnDayRender(TableCell cell, CalendarDay day) +385
   System.Web.UI.WebControls.Calendar.RenderDays(HtmlTextWriter writer, DateTime firstDay, DateTime visibleDate, CalendarSelectionMode selectionMode, Boolean buttonsActive, Boolean useAccessibleHeader) +3071
   System.Web.UI.WebControls.Calendar.Render(HtmlTextWriter writer) +1019
   System.Web.UI.Control.RenderControlInternal(HtmlTextWriter writer, ControlAdapter adapter) +27
   System.Web.UI.Control.RenderControl(HtmlTextWriter writer, ControlAdapter adapter) +99
   System.Web.UI.Control.RenderControl(HtmlTextWriter writer) +25