Erev Yom Kippur Kol Nidre service.  Many of us wear white for Rosh HaShanah and Yom Kippur services, as a symbol of seeking His purity.

Tishrei 10. Yom Kippur is considered the holiest day of the year. In Bible times, it was the only day the Cohen HaGadol, the High Priest, went into the Holy of Holies and procured forgiveness for all sins. Lev 16:22 "The goat will carry on itself all their sins to a solitary place; and the man shall release it in the desert." After the ceremonies were completed, the Aaronic benediction was pronounced using the sacred Name of G-d. It was the only time in the year that the Name was actually verbalized.