Been sailing in the BVI since 1969. Had my own boat there for 11 years, and spent many Christmas's there.
The so-called Christmas winds are not really predictable within a narrow window. They don't show up every year. When they do, they generally blow for a few days, then are gone.
When they do blow, they often blow quite hard. Expect 30-40 knot
winds or more; once-in-awhile more than that. I took a trip from the BVI direct to Grenada
(416NM) in the early 90's. The Christmas winds blew like the very devil, and for a much longer time than usual. We had 45 knot
winds most of the trip, and during the next month coming up the Windwards and Leewards we had relatively "normal" days as well as "Christmas Wind" days. Last day in the Leewards we had 50knots all night while anchored; crossed the Anegada passage
with 30-35 knot winds making 7-9 knots under furled genoa
alone -- seemed almost light.
But that's NOT the norm. Most years, as I said, the Christmas Winds come for a few days, blow like the devil, then depart. Makes for a good time to go pub crawling or just hunker down in a snug anchorage.