At what event do theater actors try on their costumes for the first time? First read-throughs Auditions Fittings Dress rehearsals

Fittings!

At first read-throughs you get the script for the first time.

At auditions you are just trying out for the part.

At dress rehearsals you are practicing the entire show from start to finish, so you will already have your costume.

Fittings is were actors try on the first costumes