Test mode is test the general settings of a calendar not to preview for future months. It only works to test on a day of the current month we are in when testing.
For example if on November 30th you tried to test for December 1st you would see what is being displayed on November first as we are still in the 11th month.
If you then on December 1st went into test mode you would be able to see on any day in the 12th month.