Well it's the date I've said when all the people ask and that's when Steam also gives greenlight, so it's as simple as that really.
That makes it sound like you picked a date you can't honestly keep, in terms of releasing a working, bug-free game with achievements. So, I'd say it is more honest to move back the date, so you can release a game everyone will be happy with. A whole lot of successful games have pushed back their release dates, often at the last minute--it's totally common in the industry . People complain, but they're still happy when the game does come, and it does well.