Is the night of power (laylat al-qadr) fixed?

Is the night of power (laylat al-qadr) fixed, year to year, or does it change? I've been reading some hadith and the only conclusion I can come to is that whenever it is revealed, to be on such-and-such a date (last ten odd nights, 27th, or the 23rd/25th/27th) it's **that date for that year** and then it changes. Is that correct? Also, related: What is the cause of the night of power? Is it the angels taking records back to heaven, or is it the mark of when the Quran was first revealed, or did the two just coincide once?