Halakhah su Levitico 23:32

I also found with regard to the Day of Atonement “from dusk until dusk you should observe your Sabbath” (Leviticus 23:32). Furthermore, it is written “on this very day” (ibid. 23:29,30) there is cutting off of anyone who works on it or of one who eats, there is no difference whether this took place during the night or the day, for “very day” begins with dusk.

Here are honest witnesses that the day begins with dusk. Similarly for all the holidays and the Sabbath, for all are “appointed seasons of God, holy gatherings” (ibid. 23:4). Only the Sabbath is called “a Sabbath for God” (Exodus 20:10, Deuteronomy 5:14), for God rested during Creation. Since both the year and the day are dependent on the sun, for both motions are similar one to another, therefore the seventh year is comparable to the Sabbath day. Hence it is also written with regard to the seventh year “a Sabbath for God” (Leviticus 25:2). Therefore, just as the Sabbatical year begins with the autumn season, so the beginning of the Sabbath day is in that period of the day corresponding to autumn, which begins with dusk.

The Gerrer Rebbe (in a responsum that appears in Piskei Teshuvah 252, which was published in 5697) discusses one who embarks on Motza’ei Yom Kippur and reenters Yom Kippur in the air. He writes that on a biblical level, one is not obligated to resume fasting. He bases his assertion on the pasuk (Vayikra 23:32) that presents the obligation to fast on Yom Kippur as "meierev ad erev," (from evening to evening). Thus, it seems that one does not observe Yom Kippur unless he was in that location in the evening at the beginning of the fast. The Gerrer Rebbe, though, implies that rabbinic law requires one to resume fasting if he has re-entered Yom Kippur. This does not imply that rabbinic law requires one to fast upon reentering other fast days, since these fasts are not rooted in biblical law and are not treated nearly as strictly as Yom Kippur.