The special after-hours session, which will not include a vote, was triggered by a request submitted by Liberal foreign affairs critic Marc Garneau last week.
The debate gets underway at 6:30 p.m. ET, and could continue until midnight, depending on how many MPs want to speak. CBCNews.ca will have live coverage.
An answer on which ministers, if any, would speak on behalf of the government was hard to pin down for much of the day, but the PMO said late Tuesday Immigration Minister Chris Alexander and Employment Minister Jason Kenney would speak, and James Bezan, parliamentary secretary to the minister of national defence, would be present as well.
NDP Leader Tom Mulcair did manage to get the government to reveal the scope of the current deployment — 69 special advisers — in a response by Prime Minister Stephen Harper in question period Tuesday.
But Foreign Affairs Minister John Baird later clarified in an interview with Evan Solomon, host of CBC News Network's Power & Politics, that Harper has authorized "up to" 69 advisers, but not all are in place yet.
With that question answered, NDP foreign affairs critic Paul Dewar will likely shift his focus to more wide-ranging questions on Canada's efforts in Iraq.
"We would hope to address what the goal of the mission is — something the defence minister has refused to address head-on," he told CBC News.
"We have asked this question ever since I returned from a trip to Iraq with [Foreign Affairs] Minister [John] Baird, where we were asked for humanitarian aid — not military support."
During Baird's appearance before the House foreign affairs committee last week, Dewar said, the foreign affairs minister "agreed with the NDP that current humanitarian assistance in Iraq is insufficient."
"He also agreed to NDP proposals that Canada should support refugee camps, victims of sexual violence, protections for religious and ethnic minorities, and the prosecution of alleged war criminals," Dewar said, arguing Baird has yet to follow through on the commitment to boost "humanitarian and governance assistance" to Iraq.
"It's not surprising the government can't tell us how long our military involvement in Iraq will be if they can't explain what we're doing there in the first place," Dewar concluded.
Garneau told CBC News that, even without a vote, he thinks it is a "good thing" to give MPs the opportunity to weigh in on the subject.
"It's the essence of democracy."
Garneau said the Liberal Party supports the "initial plan," based on what the government has said so far — which he describes as a "non-combat, to be reviewed in 30 days, sending in special forces to advise sort of thing." But he still has some questions.
Garneau intends to use his speaking slot to talk about the "time scale" underlying any decisions on a continuing role for Canada.
"To defeat ISIS is going to take a long time, so I want to sort of bring people's thinking around to a different time scale," he told CBC News.
"This is going to be a long-term endeavour, and we have to be very careful about making sure that we are comfortable, and Canadians are comfortable, with any decisions along the way to change the role, other than pulling out entirely."
He'll also talk about ISIS, "and why it concerns us," he said.
"We have to go into this with our eyes wide open, it's not just 'Let's commit for 30 days, and then rethink it without any idea on how it can develop.' This kind of debate will, I hope, make people realize that this will potentially take a long time, and could develop in a lot of different directions, depending on how many other countries get involved."
May warns against 'simplistic debates'
Green Party Leader Elizabeth May said religious extremism is not the only problem.
"The threat is because these brutal terrorists are armed to the teeth. Why has Canada not signed the Arms Trade Treaty, to control the sale of conventional weapons — guns, tanks, rocket launchers?"
She also believes that Canada "should help in any way we can with compassionate humanitarian assistance" in both Iraq and Syria.
"The rise of Islamic militants didn't come out of nowhere," she pointed out.
"The history is a mess, starting with the U.S. creating al-Qaeda and recruiting bin Laden to take on the U.S.S.R. in Afghanistan. This is not a comic strip with good guys and bad guys. Simplistic debates and simplistic solutions could make matters worse — create martyrs and deepen the attraction to ISIS ranks."