The Hunger Games opened to an estimated $19.74 million at midnight, which ranks seventh all-time behind the last three Harry Potter and Twilight movies. It did score the top midnight debut ever for a non-sequel, and it earned more than twice as much as the first Twilight movie ($7 million).
If one is to use the opening pattern of Breaking Dawn Part 1 and New Moon (which also opened outside of the Summer), The Hunger Games would be poised for a three-day take between $90 and $110 million. Since those were sequels, though, it's probably fair to assume The Hunger Games will at least hold up a little bit better over the next few days.
There's also a very distant chance that its pattern winds up closer to that of The Dark Knight, which would mean a weekend gross of around $160 million; it's been nearly four years since that movie came out, though and midnight attendance patterns have changed significantly in that time.
Regardless, The Hunger Games is off to a fantastic start, and it still has a very legitimate shot at passing Alice in Wonderland's $116.1 million debut to set a new March and non-sequel opening weekend record.