Tekiah! Teruah! Shevarim! Tekiah Gedolah!
In the traditional liturgy, the special character of each holiday is particularly conveyed by the piyyutim (hymns, liturgical poems) that are recited or chanted on that day. Most of these piyyutim have been omitted in Reform liturgies since the nineteenth century, out of a sense that their Hebrew diction is too arcane and their theology too medieval. Yet, some of these poems have routinely been retained in Reform High Holy Day prayer books, particularly for Yom Kippur.
There are many elements which make the High Holy Days a unique experience. Often, congregations swell to double or triple their usual size, the musical settings of even common liturgy are different, and some might alter their dress by wearing either traditionally all-white garments or more formal wear than they would sport on Shabbat. Some congregations even have unique garments to dress their Torah scrolls in white.
If you attended worship services at a Reform congregation anywhere in North America during the last month or so, chances are good you heard a sermon about Mister Rogers.