Object reference not set to an instance of an object - C# (191924)

private void generateTimeTable_OneDay()
{
   oneDay = new Classes.Day[5];
   
   for (int i = 0; i < 5; ++i)
   {
      oneDay[i] = new Classes.Day();
      oneDay[i].number = i + 1;
      oneDay[i].sumject /*sumject?*/ = item[i].subject;
      MessageBox.Show(oneDay[i].number.ToString());
   }
}