The 33rd mitzvah is that the kohanim are commanded to don special garments for dignity and beauty, and only thereafter to serve in the Temple.

The sources of this commandment are G‑d's statements1 (exalted be He), "Make sacred garments for dignity and beauty for your brother Aaron"; "Bring forth his [i.e. Aaron's] sons and dress them with the tunics."

The priestly garments referred to are the eight worn by the kohen gadol and the four worn by a regular kohen. The kohen's service in the Temple is invalid in any case where he did so wearing either fewer or more than the garments designated for him2 for that service.

The kohen who performs the service lacking the proper garments3 receives misah bidei Shamayim (a Heavenly death sentence). Accordingly, we see him listed in the end of tractate Sanhedrin4 among those who receive this punishment.

Scripture does not state this explicitly, but states,5 "You shall dress them with belts [and with hats]...and with this they shall be kohanim." Our Sages explain6 that "As long as the kohanim wear their garments, their priesthood is intact, but when they are not wearing their garments, it is as if they are not kohanim." As we will explain later,7 a non-kohen who serves in the Temple receives a [Divine] death sentence.8

The Sifra says regarding the verse,9 "And he placed upon him the breastplate": "This passage teaches us the law for that time and for all subsequent generations; for the daily service and for Yom Kippur. For the daily service he wears the garments that contain gold, and for the service of Yom Kippur he wears the white [linen] garments."

We can see from the wording of [another passage of] the Sifra that the wearing of these garments is counted as a positive commandment. It states,10 "Which verse teaches us that Aaron should not wear these garments for his own glory, but rather as one who fulfills the king's decree? The verse,11 'And he did exactly as G‑d commanded Moshe.' " This means that although these garments are exquisitely beautiful — made of gold, emeralds, rubies, and other precious stones — his intention should not be to adorn himself, but solely to fulfill the commandment given by G‑d to Moshe, i.e. to constantly wear these garments in the Temple.

The details of this mitzvah are explained in the 2nd chapter of tractate Zevachim,12 and a number of passages in tractates Yoma13 and Sukkah.14