Definition of dress rehearsals:
(n) :
(theater) A rehearsal prior to a public spectacle, where all the performers are in costume, in order to test all aspects of the production by closely reproducing the actual conditions of a performance.
(n) :
The final rehearsal before a performance, whether or not costumes or performance dress are worn.
(n) :
(by extension) Any complex activity that serves to prepare for a more important iteration of it later.