When is the best time to reread a difficult passage in a text?
right after reading it the first time
after reading all of the remaining text
when rereading the whole text a different day
after reading something besides the text

The answer is: right after reading it the first time

Difficult passage often contain information or point of view that cannot be easily understood by most people.

After reading the first time, most people would have a rough idea/understanding regarding what the writer is trying to say with the passage. Reading it after the first time would serve as a method to make confirmation on whether our rough understanding is correct or not.