Apple yesterday urged developers to submit their Lion apps for review before the OS X upgrade launches, making some wonder whether the operating system will launch this week as many had expected.

In an email, Apple told registered developers to "Submit your Lion apps for review now so they can be on the Mac App Store when Lion ships this month."

Apple has kept mum about a release date for Lion, otherwise known as Mac OS X 10.7, saying only that it would ship this month. In the email to developers, Apple said the upgrade would be available "soon."

It's unclear how long it will take Apple to review new applications that support Lion for inclusion in the Mac App Store.

The typically secretive company does not usually talk about the approval process for its app stores, but at the Worldwide Developer Conference in June 2010, CEO Steve Jobs said, "Ninety five percent of all the apps we get submitted are approved within seven days."

Apple has never disclosed a similar statistic for the Mac App Store, which launched in January 2011.

The late call for developers to submit their Lion applications for review may mean that Apple will not launch the upgrade on Thursday, as several Apple blogs have reported. In any case, Apple has historically preferred to launch its operating systems on Friday.

For example, Snow Leopard went on sale Friday, August 28, 2009, while Leopard hit the shelves Friday, October 26, 2007. In fact, since 2005's Tiger, Apple has issued operating system upgrades only on a Friday.

A wild card in this edition of Apple's release game is that Lion will be distributed only as a digital download through the Mac App Store, giving the company more flexibility when it ships the product because it does not need to duplicate physical media, package the product and deliver it to brick and mortar stores.

On July 1, Apple provided a "gold master" build of Lion, the label some developers use for software that has been completed and presumably is ready to ship, and has not seeded them with another version since.