The work stoppage should occur at the country's main fields in the off shore Campos basin regardless of whether Petrobras presents a proposal before then.
The Campos basin offshore São Paulo state accounts for more than 80% of Brazil's crude output of 1.8 million barrels per day. Petrobras has 42 oil rigs in the area.
"The decision has been taken to only negotiate with the strike on," said José Genivaldo Silva, director of the United Oil Workers' Federation.
The union is demanding that Petrobras count the day workers leave the platform for the shore as a working day.
Petrobras Chief Executive José Sergio Gabrielli said the company was preparing a contingency plan and that it remains open to negotiations.
"We are negotiating, we are open, and we're of the nature to avoid the strike so that it doesn't affect production," Gabrielli told journalists in São Paulo on Friday.
"Production will continue. We will, if needed, implement a contingency plan to maintain minimum staffing to continue producing," he said.
Worries about the potential strike helped push up world oil prices on Thursday, propping it to new record on Friday.
However the conflict could extend as of Tuesday to a five-day nationwide strike at all Petrobras facilities, including refineries and terminals, to demand a bigger share of company profits for workers.
Mercopress