It doesn't compare to the incredible midnight debuts of the last few Harry Potter and Twilight movies, but The Avengers still registered a pretty remarkable $18.7 million at midnight. That's the eighth-highest midnight opening ever, and it edges out The Dark Knight ($18.5 million) for best superhero midnight debut in history.
While the first weekend of May has historically been a great time to open tentpole comic book adaptations, high school and college students are still in class and don't tend to turn out in huge numbers at midnight. Prior to The Avengers, the best tally for a Summer kick-off title was Spider-Man 3 at $10 million, and Iron Man 2 only earned $7.5 million in its midnight debut (that's also the previous best tally for an Avengers movie).
If The Avengers follows the pace of Spider-Man 3, it would wind up with an opening weekend over $280 million. The Iron Man 2 pace translates to a $320 million debut. Those are both outrageous figures, though, and it's hard to imagine that theaters are even capable of handling that sort of demand. It's more reasonable to expect The Avengers to perform in line with The Dark Knight and get to at least $160 million for the weekend.
On a related note, The Avengers passed $300 million overseas yesterday (its ninth day in theaters). With Russia and China opening with the U.S. this weekend, it's entirely possible that The Avengers's worldwide (domestic plus foreign) total approaches $600 million by Sunday.