It looks like Delta is targeting some United hubs with this year-round deal to Brazil. Flights are available for departure as soon as today and travel is bookable daily until the end of the schedule in April, no minimum stay required. That's about as wide open a sale can get, with very little restrictions! Saving several hundreds of dollars off the going rates too.
As an example, I found seats departing SFO on Friday, September 7, and returning from GIG on Saturday, September 15, 2018. Other dates are widely available.
As mentioned before this is an unadvertised sale and could disappear at any moment. If any of these super fares strike your fancy, I'd recommend booking now as Delta allows free cancelation within a 24-hour period if you happen to change your mind overnight.
Earlier this year Brazil introduced an e-visa system making it much easier and cheaper for U.S. Citizens to visit. - Find out more about the new e-visa here.