The Druid Circle of Dreams gives, at 14th level, the Walker in Dreams feature (XGtE, p. 23). I'm unsure about when it is used. It states, "when you finish a short rest" and "once you use this feature, you can't use it again until you finish a long rest".
The game allows for two or three short rests (not sure exactly the number) between long rests. Do you pick one short rest and use the feature immediately after you finish it? Do you get to use it any time after you finish the short rest?