By Tuesday afternoon (around 15h Eastern USA), that’s about 21h Greenwich, the prices on flights have settled. Fares go back up on Friday.

As for the flights themselves, the cheapest fares tend to happen on Thursdays. Airlines know that vacationers tend to start flying on Friday night or Saturday morning, so those fares tend to be higher.
Ditto for peak-season. If you’re going to Italy and want to save on all prices, plan to visit in April or November, not in July or August.

The exact time to book your flight:

 
According to priceoftravel.com if you’re going from North America to Europe, 7 to 16 weeks out tends to be the best window to South America, 5-16 weeks, and to Asia, 8-20 weeks out.
Going somewhere for Christmas? Buy your ticket in mid-August.

priceoftravel.com

FareCompare.com

Special: Flights from Brazil to Europe
https://www.farecompare.com/flights/Sao_Paulo-SAO/europe/cityzone.html#quote?quoteKey=CSAOCLON20180619R20180626P1CTF