What was the specific date that Tom Robinson died in Harper Lee's To Kill a Mockingbird?
We can deduce the exact date of Tom Robinson's death based on context clues author Harper Lee leaves for us in chapters 24 and 25 of To Kill a Mockingbird.
Chapter 24 opens with Scout having been invited to join Aunt Alexandra and her missionary circle for refreshments while Jem is off teaching Dill how to swim at Barker's Eddy. The first important clue we are told at the start of this chapter is that summer is almost over and "Dill would be leaving for Meridian tomorrow." It is on the exact same day Scout announces Dill's upcoming departure that Tom Robinson is killed. More importantly, we know the day takes place at the end of August since Scout also describes, "August was on the brink of September." On the same day in August that Scout joins the ladies for refreshments, Atticus rushes home and interrupts the meeting to inform Calpurnia and others of Robinson's death.
Later, in Chapter 25, Dill has already left for the summer, and Scout is flashing back to her last moments with Dill, particularly his description of Helen Robinson's reaction to the news of her husband's death. Chapter 25 begins at the start of September, as Scout describes, "September had come, but not a trace of cool weather with it." Scout also informs her reader that Dill had "left us the first of the month," which means that Dill left the first of September. But, remember that in Chapter 24, we were informed on the day of Robinson's death that Dill would be leaving the very next day. Since August has 31 days, we can only logically conclude that Aunt Alexandra's missionary circle meeting and Robinson's death both took place on August 31st.
We also know the exact year of Robinson's death since his death took place soon after his trial. During his closing remarks to the jury, Atticus references the current year as being 1935 when he reminds the jury that, while not all people are truly created equal in the optimistic sense people would like to believe, they are all equals in the courtroom.
Hence, we can deduce that Tom Robinson was sadly and unjustly killed on August 31, 1935.