It's tough to pin this fare down because the travel dates vary across the board by month. You will have the most luck traveling roundtrip in September at this price, but other limited days are available in additional months if you play around.
The best way to do that is by searching for one-way fares listed at $67 and piecing together an itinerary of two separate tickets.
As an example, I found seats searching on AA.com on flights departing ORD Tuesday, September 17, and returning from EYW on Saturday, September 21, 2019.
Need a place to stay in Key West? Here are the best deals for the mentioned dates above on Hotelwatchdog.