And was it only our local "custom" to announce any events to happen at the end of the year 1999 as of "New Year's Eve 2000?"

I suspect not. New Year's Eve being (teaching grandma to suck eggs) a conventional contraction referring to the 'evening before' any event (like All Hallow's Eve), it seems to fit. I haven't seen it being used for the entire period leading up to the New Year, though, only for the day before.