Because a Facebook event that's set to cover the whole of the 13th of December shows itself as:
(presumably because Facebook says "It starts at the end of that first day, and carries on until the end of that second day)
and Google Calendars interprets that as "Starts at the beginning of that first day and carries on until the beginning of the second day", meaning that it shows up as an all-day event for the 12th.
I _think_ that Google's logic is the correct one here, and a brief reading of ical discussions indicates that that's right. I suppose I should raise a defect for Facebook.
All of which doesn't change the fact that I went all the way into town and back just now, and that I can't see them tomorrow because I'm booked in to see The Hobbit with Julie!
Original post on Dreamwidth - there are comments there.