We had all the tracks leaked on Twitter from datamining for Mario Kart 8 Deluxe's booster course pass wave 3 DLC so I am going over what these tracks are and how reliable the leak is as Nintendo have not actually confirmed this but from what we have seen it is highly likely to be somewhat accurate at predicting a lot of the tracks.
I predict that it will release around christmas time as that is only a few months away now and it may have a christmas themed track in the mix so it would make sense for Nintendo to release it then!