The 101 MPs are expected to meet with leader Thomas Mulcair to devise a strategy for the upcoming session of Parliament.
The House of Commons resumes sitting on Sept. 17
Mulcair, a Quebecer, was elected to lead the Official Opposition in March.
The retreat comes on the eve of a provincial election in Quebec.
More than half of the NDP caucus are MPs from Quebec.
Polls are suggesting the sovereigntist Parti Quebecois is in the lead as voters head to the polls today.
(The Canadian Press)
Also on HuffPost