You can get mid-winter conditions, but most years it will be "spring skiing" type conditions by Mar 9. That means average highs around 40-45 degs or warmer, somewhat slushy, rain possible. I was at Okemo (20 miles from Kil) around March 9, 2006 and it was much like what I just described. It can be great fun if you get snow or sun, moguls get soft, outdoor BBQ grills fired-up, crowds are down weekday and weekends, usually trail counts are at about 90%, which means 100+ runs still open at Kil.
I think best time at Killington is late Feb, or any weekday.