Athens is 5 hours ahead of Brasília
Best overlap: 8:00 AM to 1:00 PM in Brasília is 1:00 PM to 6:00 PM in Athens.
The overlap falls at 8:00 AM to 1:00 PM in Brasília, or 1:00 PM to 6:00 PM if you're in Athens. You've got a decent 5-hour window. Set a recurring time near 8:00 AM in Brasília (1:00 PM in Athens) and stick to it. Heads up: because Athens changes its clocks and Brasília doesn't, the difference moves by an hour twice a year. Confirm the overlap around the DST switch.