Like Shabbat and holidays, Yom Kippur is called “a sacred occasion” (mikra kodesh), as we read, “The tenth day of this seventh month is the Day of Atonement. It shall be a sacred occasion for you” (Vayikra 23:27). With regard to Shabbat and festivals, this means enjoying and honoring them, as our Sages say (Sifra, Emor 12:4), “How does one sanctify a festival? With food, drink, and clean clothes” (codified in MT, Laws of Yom Tov 6:16; SA OḤ 529:1). However, we cannot say the same for Yom Kippur, when we must fast. Thus, our Sages (Shabbat 119a) expound the verse “The Lord’s holy day (should be) honored” (Yeshayahu 58:13) as referring to Yom Kippur. “There is no eating and no drinking on it, so the Torah says: Honor it with clean clothes” (Rif; Rosh on Yoma 8:9).
Many people, men and women, wear fine, elegant white clothes on Yom Kippur, imitating the ministering angels. On Yom Kippur we are not seduced by temptations of the flesh; we are sin-free, like the angels. Women, even those who do not wear white, avoid wearing jewelry and fancy clothes on Yom Kippur, due to the dread of impending judgment (Mordekhai; Rema 610:4; MB ad loc. 16-17). Many Ashkenazic men wear a white cloak called a kittel, which, in addition to being angelic, is also similar to a burial shroud. The kittel reminds us of death, leaving us contrite and humble, and inspiring us to repent. A kittel must be removed before using the bathroom, unless one is only urinating (Mateh Ephraim ad loc. 12; MB ad loc. 18).
It is a mitzva to clean the house before Yom Kippur and to cover the table with a nice tablecloth, just as one does before Shabbat (Mordekhai; Rema 610:3; AHS ad loc. 2). There is also a mitzva to take a shower in honor of Yom Kippur, just as one does for Shabbat. Some immerse in a mikveh as well (5:10 above).
It is a mitzva to clean the synagogue and prepare it for Yom Kippur so that it will look its best. There is also a mitzva to leave on all the lights in the synagogue in honor of the day, as we read (Yeshayahu 24:15), “Therefore, honor the Lord with lights” (SA 610:3-4; MB ad loc. 9).
On Tisha Be-Av, which is a mournful day commemorating the destruction of the Temple, we make a point of not smelling aromatic spices (SA 559:7; SHT 556:1). In contrast, on Yom Kippur some make a point of smelling aromatic spices and reciting the berakha over them. Since Yom Kippur is a festival, there is a mitzva to honor the day in any permissible way.