It was seen in the case of Hearts of Stone DLC, that a trailer for the same was released one month prior to the roll-out of the expansion pack. As for the Blood and Wine DLC, rumors have it that it will be rolled out within the first half of the ongoing year.
Now, it is already the month of May and the developers are yet to come forward with significant details about the expansion pack and thus it is unlikely that it will be released before the month of June.
In that case, if the developers follow the same procedure as they did in case of the Hearts of Stone DLC, the trailer for the Blood and Wine expansion pack should hit the internet anytime within the ongoing month.
Thus it is being assumed that CD Projekt RED might release the first trailer for the upcoming expansion pack on May 10.
It has also been reported that the developers focused on the development of the Blood and Wine DLC only after the Hearts of Stone DLC was rolled out. This was done due to the fact that the developers abstain from indulging in multiple projects simultaneously.
Stay tuned for more update on The Witcher 3.