This typically occurs either after the table read or during the 1st and 2nd rehearsals. It is exactly what it sounds like. The Costume Designer takes measurements of all the actors so that they can accurately find/make costumes for them. The Costume Designer will usually have a sheet with all the required measurements they need. The measurements typically include: Shirt Size/Dress Size Head Circumference Chest Size Arm Length Waist Size Inseam Length Shoe Size
Once the Costume Designer has all the measurements then the next step is to make sure it all fits. After the Costumes are procured by whatever methods (sale, stock, bribery, extortion, theft) the actors need to try them on and make sure they fit properly. You'd be surprised that even with measurements previously taken some things can change, actors can gain or lose weight, or maybe the initial measurement wasn't entirely correct. This typically occurs during rehearsals as tech day approaches.
Once you have all the measurements, all the costumes acquired (legally of course) and all the actors have been correctly fitted then it's time to have your costume parade! This usually happens on tech day or right before the first run-through with costumes. The Costume Parade is pretty simple, the actors dress in their costumes and one by one "parade" across the stage to be viewed by the Director and Costume Designer. They may also turn on the show lights to see what the costumes look like under the lights. It is the chance for the Director to see Truth be told the costume parade is becoming a bit antiquated, and fewer and fewer theaters are doing it. It may still be done at large theaters or for pieces that require large grand costumes. With more and more plays set in modern times, the costumes are becoming less and less elaborate.