I know we will find out in a week or two, but here is my theory:

In the first episode, Cal brought Chloe into their room, and pointed out a miniture sailing boat. Cal then said he had rented a boat, and that they might be able to sneak out one day to go sailing.

Possibly, since they didn't do it in the other episodes, and that the wedding rehearsal is the last possible day before the wedding, they decided to sneak out then, and without telling anyone, so they wouldn't be convinced to come back to the rehearsal to go sailing.