The eve of Yom Kippur is identical [in practice] to the day. What is forbidden on Yom Kippur? Labour, eating and drinking, washing, anointment, wearing [leather] shoes, and sexual relations. However, only labour and eating and drinking are punishable by Karet, (divine excommunication).
Any laborous activity for which one is liable on Shabbat, one is liable on Yom Kippur; anything which is forbidden on Shabbat without liability is also forbidden on Yom Kippur - except that when it is done intentionally on Shabbat one is punished by stoning, and on Yom Kippur one is punished by Karet, (divine excommunication). Anything which may not be handled on Shabbat may not be handled on Yom Kippur - it is permitted to prepare vegetables and crack nuts after mincha time on weekdays, but now we tend to forbid this. Rama: If there is a fire on Yom Kippur, one can save one meal's worth for night-time, just as on Shabbat one can save for the afternoon meal. In chapter 334, there are more details on how this is applied today, and this applies to Yom Kippur too. Children have a custom to play with walnuts, and one should not stop them, even before the afternoon, and this custom came from the rule of cracking nuts mentioned above.