Marking the last gasp of summer B.O., Labor Day weekend typically isn’t a big moviegoing period. The weekend’s three wide releases, “Apollo 18,” “The Debt” and “Shark Night 3D,” aren’t likely to break out of the low-teens, so this year’s holiday frame will likely follow the pattern.

Summer box office, at $4.25 billion domestically, still trails $4.34 billion high. But most B.O. observers are optimistic that even a modest weekend will give the B.O. enough oomph to push 2011 summertime totals ahead of 2009.

The Weinstein Co.’s found-footage style sci-fi thriller “Apollo 18” could wind on top, according to bizzers, with a projected debut between $11 million and $12 million through Monday. That may be enough to outdo current B.O. champ “The Help,” which has topped domestic charts for the past two weeks. If “The Help” drops 25% (pic’s hold could be even stronger), then that puts “Help” just above $10 million for three days.

There’s not much fresh fare to tempt overseas auds this weekend, so “The Smurfs,” with an international tally of $258 million, could land on top for its fourth consecutive week. “Final Destination 5,” which came close last weekend to eclipsing “Smurfs,” expands to markets including Australia and France.

Domestically, B.O. pundits say whichever film hits around $12 million for the three days could end up on top.

Neither “Shark Night 3D” nor “The Debt” is expected to hit that, however. Relativity Media’s “Shark Night,” from Incentive Filmed Entertainment and Sierra Pictures, is projected to tally $10 million at best, while the most optimistic expectations for Focus Features’ “The Debt” have the film grossing approximately $8 million in six days.

Focus launched “The Debt” on Wednesday, taking in a reported $958,670 from 1,786 locations. The remake of an Israeli film was originally a Miramax pic.

The distrib has used an early Labor Day weekend opening as a launchpad before — last year, Focus released “The American” midweek, picking up $1.7 million on Wednesday, and in 2005, “The Constant Gardener” bowed to $929,829 on opening day. “American” earned $19.8 million in six days; “Gardener,” $12.7 million.

“The Debt,” directed by John Madden, toplines Helen Mirren and Tom Wilkinson as retired Mossad agents. Jessica Chastain and Sam Worthington also star as younger versions of those characters, set in 1966 West Germany. Pic expands today to 1,826 locations but will have to share adult auds with “The Help.”

Both “Shark Night” and “Apollo 18” will compete for filmgoers under 18. With nearly 90% of its 2,500 locations in 3D, “Shark Night” will see a bump from premium ticket prices.

“Shark Night” was co-financed by Incentive and Sierra for a reported budget in the low-$20 millions, with Relativity footing P&A costs. Companies have limited exposure, since Sierra widely pre-sold rights internationally.

Meanwhile, Weinstein’s “Apollo 18,” produced by Timur Bekmambetov, cost in the low-single digits, according to a source close to the project. Pic, which pieces together found footage from the failed NASA mission, bows this weekend in Russia and the U.K.

In limited release, Lionsgate’s Latino-targeted division Pantelion bows “Saving Private Perez” at a gutsy 161 locations. Likewise, IDP/Samuel Goldwyn has high hopes for raunchy comedy “A Good Old Fashioned Orgy,” launching the film Stateside at 143 engagements.