Shulchan Arukh, Orach Chayim שולחן ערוך, אורח חיים Shulchan Arukh, Orach Chayim 602:1-604:1 שולחן ערוך, אורח חיים תר״ב:א׳-תר״ד:א׳

The order of the esrei yemei teshuva (ten days of repentenance), and in it is 1 paragraph.
Rema: We fast the day following Rosh HaShana (Tur) and it is a Public Fast. Everyday between Rosh HaShana to Yom Kippur, we increase prayers and supplications. Rema: And we say Avinu Malkeinu every evening and every morning except for Shabbos (Tur). And even if there is a bris milah, so that we do not say Tachanun, we say Avinu Malkeinu (Minhagim). And each day we say Vidui 3 times before daybreak except for the eve of Yom Kippur, that we do not say it except one time (Rokeach). And we do not place anyone into Cherem, and we do not have a person swear in Bais Din until after Yom Kippur (Maharil). And we do not sanctify the New Moon until Motzei Yom Kippur. And the Shabbos between Rosh and Yom Kipper the custom is to say Tzidkatkhah.

Even if one whom is not careful regarding non-Jewish bread [during the year], should be careful (during the ten days.) Rema: And there is to every person to search and inspect in their deeds and return from them in t'shuvah (repentance) during the (Ten) Days of T'shuvah. And a doubtful aveira (sin) needs more repentance from a certain aveira, because one that knows regrets more than one who does not know (which is the case when one is unsure). And therefore the Conditional Asham Offering needs to be more valuable than a Guilt Offering (Rema's own thoughts and Rabbeinu Yonah, beginning of Brachos).

“The order of the Eve⁴All Jewish days, holidays, and festivals begin and conclude at sundown. The “Eve” of a holiday is considered any day which precedes the sundown marking the beginning of that holiday. of Yom Kippur⁵Yom Kippur, or the Day of Atonement, falls on the tenth day of what, during our time, is the first month of the Hebrew calendar, the month of Tishrei. During the biblical period it was referred to as the seventh month. Yom Kippur follows by ten days the actual Jewish New Year, or Rosh HaShanah, which falls on the first day of the month of Tishrei. Rosh HaShanah and Yom Kippur are called the Days of Awe, Yamim ha-Noraim, because of their demand upon a religious Jew for special piety and self-reflection during this period of time devoted to repentance and self-improvement. These days which usually occur in the Fall of the year (September-October) and are referred to as the High Holydays. Yom Kippur is considered the most sacred day of the entire Jewish year and its commemoration involves a special, unique set of laws which are enumerated in the body of this thesis. For the biblical ordination of this holiday see: Leviticus 16 (all, but especially 16:29-34) and Numbers 29:7-11. (the Day of Atonement)” - Containing two paragraphs.
It is a commandment to eat on the Eve of Yom Kippur and to increase (one’s eating) at the meal.
Hagah:⁶Hagah, הגה, introduces the notes added to the text of Joseph Caro by Rabbi Moses b. Israel Isserles. Isserles is also known as “the Rema”, an acronym for Rabbi Moses Isserles. He was a Polish rabbi, codifier and halakhic authority, who lived from 1525 or 1530 until 1572. He was born in Cracow under the name of Isserel-Lazarus which was later shortened to Isserles. He studied in Lublin at the yeshivah of Sholom Shachna. Isserles obtained such a fine reputation that he became known as the “Maimonides of Polish Jewry”. Isserles was in the middle of writing a code himself following the pattern of the four Turim by Jacob b. Asher which he called Darkhei Moshe which was to assemble the halakhic material of his time in a short, synoptic form so that a dayyan, a decision maker, could more easily find the material he needed to formulate a ruling on a particular issue. In the middle of his writing of the Darkhei Moshe he received a copy of the Beit Yosef of Joseph Caro which in essence had already accomplished this goal. But Isserles decided to complete his work operating a bit differently than did Caro. Isserles did not always agree with Caro’s selection of the “three pillars of halakhic decisions”, Alfasi, Maimonides, and Asher b. Jehiel to decide issues. He followed the principle which stated that laws should be decided according to later scholars. He also often agreed with Asher b. Jehiel and his son Jacob b. Asher even when they were in the minority, unlike Caro. Also Isserles realized that Caro ignored in his work Ashkenazi practices that were very much a part of his Polish and European community but were not included in Caro’s work in the world of Sephardi Jewry.
Isserles wrote other halakhic works, such as Torat Ḥattat which focused on mainly Jewish dietary laws. Finally Isserles received the Shulḥan Arukh by Joseph Caro. Like the earlier and more extensive Beit Yosef, it lacked many halakhic contributions and customs, minhagim, of Ashkenazi Jewry and was therefore not an adequate code for this segment of the Jewish world. Isserles decided to add notes (hagah or hagahot) to the body of the Shulḥan Arukh which has also been referred to as the “mappah” or “tablecloth” over the “Shulḥan Arukh” or “prepared table”. In his glosses, Isserles added his conclusions which he drew in his work Darkhei Moshe to the Shulḥan Arukh. In many cases he disagreed with Caro and he stated his disagreement, or he would cite an Ashkenazi custom not found in Caro’s work. He maintained the brief style employed by Caro, and he provided the existence of differing points of view by later scholars and Ashkenazi Jews which he felt were needed for a dayyan to be able to arrive at a correct decision. He often modified the views of the meḥabber, the author, as he referred to Caro, he explained, contradicted, added to and refined the structure.
Isserles put much emphasis on the custom, the minhag. He often gave it the same force as the halakhah. If there was no halakhah in existence, or in some cases where a minhag and a halakhah conflicted with each other, he decided according to the minhag, the custom which the people actually followed in their daily lives. If Isserles disagreed with a particular custom he would state so and he would urge against following such a custom.
Isserles was very lenient in cases of stress or in cases which would involve considerable financial loss. His leniency, which was seldom found in the works of others, was the subject of criticism by many of his contemporaries particularly Ḥayyim b. Bezalel who studied with him under Sholom Shachnan in Lublin. Even in view of the extensive criticism Isserles received, his notes to the Shulḥan Arukh became accepted and his rulings and customs were binding on Ashkenazi Jewry. The mappah of Isserles made the Shulḥan Arukh of Caro acceptable to Ashkenazi Jews and the authoritative code that it remains to this day.
The Shulḥan Arukh which was first published in Venice in 1565, was first published with the mappah of Isserles in Cracow in 1569-71 and it has been a part of the accepted text ever since.
Louis Ginzberg in Menachem Elon, E. J., “Codification of Law” v. 5. pp. 628-56;
Simḥa Katz, “Isserles, Moses ben Israel”, v. 9, pp.1081-85. It is forbidden to fast on it (the Eve of Yom Kippur), even a dream fast⁷A fast was recommended to be observed as the result of an ominous dream (or a nightmare), ta’anit ḥalom, תענית חלום, to avert the evil consequences dreamt. In talmudic times and later it was believed that bad dreams could have pernicious effects. The fast was regarded of such urgency that the rabbis permitted it even on the Sabbath, but one was to fast on a weekday as well, as a repentance for having dishonored the Sabbath Joy through fasting. A fast as the result of a bad dream, though, is not to be observed on Yom Kippur Eve.
Editorial Staff, E. J., v. 6, p. 1196
(What amends shall he make (for having fasted on the Sabbath)? - He should observe an additional fast.)
B. Ta’an. 12b. (Soncino p.55)
Raba b. Meḥasia also said in the name of R. Ḥama b. Goria in Rab’s name: Fasting is as potent against a dream as fire against a tow. (Dreams were believed portents forshadowing the future, though, as seen here, the evil they foretold might be averted; cf. Ber. 55-58. B. B. 10a; Yoma 87b et passim). Said R. Ḥisda: Providing it is on that very day. R. Joseph added: and even on the Sabbath.
Shabbat 11a (Soncino p. 40, Shabbat I)
R. Eleazer also said in the name of R. Jose b. Zimra: If one keeps a fast on Sabbath (to overt the omen of a dream), a decree of seventy years standing is annulled; yet all the same he is punished for neglecting to make the Sabbath a delight. What is his remedy? R. Naḥman b. Isaac said: Let him keep another fast to atone for this one.
Ber. 31b. (Soncino pp. 194-95) (due to a nightmare), (מהרי״ל).⁸Maharil, מהרי״ל, is an acronym for Morenu ha-Rav Jacob ha-Levi. His real name was Jacob ben Moses Moellin. He lived from around 1360 until 1427. He was born in Mainz and became the foremost talmudist of his generation and the head of the Jewish communities of Germany, Austria, and Bohemia. He studied under his father and later went to Austria where he was ordained after studying under Meir ha-Levi and Sholom b. Isaac. After the death of his father, Maharil established a yeshivah from which came the greatest rabbis of Germany and Austria.
Moellin was famous and halakhic questions were asked of him throughout Europe. Many of his rulings became the foundation of the Jewish way of life for German Jewry. His decisions were characterized by the fact that he took into account the conditions of the time including the economic situation of a particular community. He would often decide to be strict in a case where a community had no rabbinic leadership. Moellin was concerned about leaders who did not possess the proper authority and the neglect of proper Torah and talmudic study that resulted from decisions arrived at using codified halakhic works instead of thorough original investigation. He placed a great deal of importance on charity and the honor of the poor.
Moellin was an accomplished ḥazzan and fought for the preservation of traditional melodies for the liturgy. His known works were two, Minhagei Maharil, (Sefer Maharil), which was first published in Sabionetta in 1556 and compiled by his student Zalman of Saint Goar, was a collection of halakhic statements, customs, and explanations which Zalman had heard from Moellin. It was these customs of Germany that Moses Isserles used so often in his glosses to the Shulḥan Arukh. The second work was a collection of responsa arranged by Eleazar b. Jacob and published in Venice in 1549.
Ephraim Kupfer, E. J., v. 12, pp. 210-11. And if one vows to fast on it, see above (in the Shulḥan Arukh, Oraḥ Ḥayyim), chapter 570, paragraph 2.⁹Oraḥ Ḥayyim chapter 570, Paragraph 2: “One who vows to fast on the Sabbath, on a festival, or on the Eve of Yom Kippur or on Ḥanukkah or Purim, the laws are the same as for one who vows to fast on so and so many days and these (holy) days happen to occur on them. If he expressed it using the word “vow”, the law is as if one has taken them upon himself, with the expression, a vow (it has the legal character of a vow). But if he expressed it merely with the (ordinary) expression of accepting a fast then the law is as if one has taken upon himself (the fast) with the expression of accepting a fast”. That is if one “vows” to fast on the Eve of Yom Kippur then he must if he uses the words “vow” and “fast”. If he only says I am going to fast, without using the word “vow”, it is not serious and he can postpone it. (Acceptance of a fast by a) vow is more serious than accepting a fast without it.”