I was there last year from 23rd dec for a week. one ruined snowboard later, because there was not enough snow!
got told that it was the first year it had been so lacking in snow tho!
Last year was a slow start from the whole of europe, i went for new year in les deux alps and there wasnt enough snow.
There are a few good runs in borovets at the top and also down to the middle gondola station these should all have enough snow on for the 23 of december as the weather forcast year predicts amazing snow conditions this year. il be there at new year so fingers crossed