When Does Yom Kippur Begin?

The Talmud (Rosh Hashan 9a) teaches that one must add a bit of time onto Yom Kippur, and abstain from melacha, and from eating and drinking and the other afflictions, a few minutes before the fast begins. There is no clear definition of this time. This misva is known as Tosefet Yom HaKippurim.

This acceptance should preferably be done verbally. Therefore, one should say "hareinu mekabel alay tosefet yom kippurim, hamisha inuyim veissur melacha" (I accept upon myself the added sanctity of Yom Kippur, including the five afflictions and melacha).

Hacham Ben Sion suggests that one should first put on his tallit, as after he accepts upon himself the fast, it is considered to be night, and one does not wear a tallit at night.