dress rehearsal

Short definition from Wiktionary

Noun

1. 1
   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.
2. 2
   The final rehearsal before a performance, whether or not costumes or performance dress are worn.
3. 3
   Any complex activity that serves to prepare for a more important iteration of it later.