Since Yom Kippur is the time of forgiveness, atonement, and acceptance of the Jews’ repentance, and since repentance requires vidui, it is a mitzva to confess as soon as Yom Kippur begins, meaning, at Ma’ariv. However, the Sages were concerned that something might go wrong at the pre-fast meal – someone might get drunk and be unable to confess during Ma’ariv or might choke during the meal and die before confessing. Therefore, they ordained that each person confess at Minḥa, prior to the pre-fast meal. Nevertheless, vidui must be repeated at Ma’ariv on Yom Kippur, as that is still the primary time for vidui and atonement. Despite having confessed at Ma’ariv, we confess again at Shaḥarit, Musaf, Minḥa, and Ne’ila (Yoma 87b). Given that we have just confessed and have not had time to sin, this would seem to be unnecessary. Nevertheless, whenever we stand in prayer before God on Yom Kippur, it is appropriate to confess, as repentance is the mitzva of the day. (This is different from the rest of the year, when even those who confess every day do not do so during the Amida.) Additionally, it is possible that one sinned after Ma’ariv, so he must confess again at Shaḥarit. Similarly, he may have sinned after Shaḥarit, so he must confess during Musaf, and so on. For Yom Kippur continues to atone until the end of the day, when it gets dark (Levush, SAH 607:1).
During each silent Amida, vidui is recited after the conclusion of the berakhot and before the recitation of Elokai Netzor. During the repetition of the Amida, it is recited within the middle berakha (the sanctification of the day). Technically, an individual is not required to confess during the repetition, though he must stand (SA 607:3). However, in practice, the Rishonim write that it is proper for the members of the congregation to recite vidui together with the ḥazan (Ran; Rema) so that everyone recites vidui ten times over the course of the day, corresponding to the ten times that the Kohen Gadol uttered the Tetragrammaton (10:15-16 below). The ten confessions are: one before the fast during the silent Amida of Minḥa, one at night during the Amida of Ma’ariv, and eight more during the four prayer services of the day: Shaḥarit, Musaf, Minḥa, and Ne’ila. In each of these four services, one vidui is recited during the silent Amida and a second is recited during the ḥazan’s repetition. The vidui that we recite during the Seliḥot after the Amida of Ma’ariv does not count, as it is not recited within an Amida (Tur 621; MB ad loc. 2).
According to Ramban, in addition to the confessions recited on Yom Kippur, one must recite the vidui after the pre-fast meal, before dark, so as to start Yom Kippur in a state of repentance. Aḥaronim write that it is proper to follow this stringency (Shlah). However, there is concern that some people will not be able to recite it because they drank during the meal. Therefore, they ordained the recitation of vidui at Minḥa as well, before the meal. The main purpose, however, is to enter Yom Kippur with the right mindset. One may fulfill this vidui by reciting the minimum: “But we have sinned, done wrong, and rebelled.” In practice, two customs have developed to fulfill the stringency right before Yom Kippur: Ashkenazim recite Tefila Zaka, which contains detailed confessions (MB 607:1), while Sephardim recite the poem Lekha Keli Teshukati, which includes confessions. There is an opinion that these prayers must be recited while standing, since they are a type of vidui (Pri Ḥadash). Nevertheless, the custom is to recite them while seated, as be-di’avad one may confess while sitting.
Those who will not attend the synagogue are not obligated to recite ten confessions. During each Amida that they pray, they should recite the appropriate vidui. At least one vidui must be made, as this is an obligation of the day. If at all possible, two confessions should be made, one at the beginning of Yom Kippur and another one at the end, at the time of Ne’ila. (See 6:3 n. 1 above.)