OSX has always released on a Friday. Nothing new with 10.5.
Neverland, Apple (almost) always releases new products on a Tuesday. (This is common practice among other retailers as well, check the publishing and music industries.)
And why they decided to release the new OS on a Friday, I don't know. If I'm not mistaken, they released the iPhone on a Friday, n'est-ce pas? Maybe they like to release their really big new products on a Friday, to emulate the hype of a movie opening? In any event, all the computer upgrade releases that I can recall arrived on a Tuesday.