1. The day before Rosh Hashanah is the last day of the year, and it is the last opportunity to repent before the Day of Judgment. It is a day of reflection on the year gone by, and a day of preparation for the coming year. The laws that apply to this day are different than those practiced throughout the entire month of Elul. Many people have a custom to fast on this day and to pray by the graves of the righteous, so that God might have mercy upon us by virtue of their meritoriousness (Ben Ish Chai, Nitzavim 2).
2. On the day before Rosh Hashanah, a person should engage in Torah study, the performance of commandments, and repentance. One should ask forgiveness at this time from those he has wronged and not wait until Yom Kippur eve (Kaf HaChaim 581:9).
3. One should wash his clothes and have his hair cut on the day before Rosh Hashanah in honor of the festival. It is also a good idea to cut one's fingernails on this day, being even more careful than usual that they not protrude beyond the flesh. Care, however, should be taken to have one's hair cut before noon (Shulchan Arukh 581:4; Ben Ish Chai, Nitzavim
4. Throughout the entire year, and especially on the day before Rosh Hashanah, when having one's hair cut a person should meditate on ridding himself of "kochot hadinim" (negative forces), and on fulfilling the precept of peyot (sidelocks). Some authorities say that one should have intention to fulfill the commandments "Do not clip your hair at the temples or mar the edges of your beard" (Leviticus 19:27), and "You must give him his wage on the day it is due" (Deuteronomy 24:15), and "Do not let a worker's wage remain with you overnight until morning" (Leviticus 19:13). One who cuts his hair at home should only have intention to fulfill the commandment not to clip one's hair at the temples, etc. (Ben Ish Chai, Nitzavim 3).
5. We put on Sabbath garments for Rosh Hashanah to indicate that we trust in the mercy of the Holy One, blessed be His name, that He will make our judgment bright as the light. Some have a custom to wear white clothes on Rosh Hashanah, but not silk or embroidered clothes (Kaf HaChaim 581:79; Mishnah Berurah ad loc. 25).
6. The prayer leader on the Days of Awe, as well as the one who blows the shofar, should abstain three days before Rosh Hashanah from anything that may cause contamination. They should familiarize themselves as much as possible with the meaning of the prayers, the liturgic poetry, and the laws concerning the blowing of the shofar. They should also study inspirational books, which tend to stir the heart of man. In fact, every person is advised to read such books before the Days of Awe (Kaf HaChaim 581:39, based upon the Zohar).
7. Although we generally do not recite Vidui (confession) in the afternoon Mincha prayer before a day on which we refrain from reciting Vidui, we do recite Vidui in the Mincha prayer on the day before Rosh Hashanah eve, because we say Selichot (penitential prayers) on the following night. If Rosh Hashanah falls on a Monday, we say "Tzidkatkha" in the Mincha prayer of the preceding Sabbath (Ben Ish Chai, Nitzavim 2).
8. On the night preceding the day before Rosh Hashanah we recite "Tikkun Rachel" and say Vidui in the Selichot. Some do not say "Nefilat Apayim" (falling on the face) after dawn (Aliyat HaShachar), and if one recites Selichot after sunrise (Netz HaChamah), Vidui may not be recited (Ben Ish Chai, Nitzavim 2).
9. A person who made a practice of saying Selichot before dawn (Alot HaShachar) during most of the month of Elul, but on the day before Rosh Hashanah was delayed until after dawn, should say Nefilat Apayim in Selichot even after dawn has arrived (see Magen Avraham 581:13).
10. We do not blow the shofar during prayers and Selichot on the day before Rosh Hashanah, neither at night nor during the day, neither with the congregation nor individually. The reason we refrain from blowing the shofar is to make a distinction between the voluntary blowing of the shofar in Elul and its mandatory sounding on Rosh Hashanah (Rema 581:3; Ben Ish Chai, Nitzavim 2).
11. If a shofar blower wishes to practice for Rosh Hashanah on the day before Rosh Hashanah, he should do so in a closed room so that the shofar not be heard outside (Ben Ish Chai, Nitzavim 2; Kaf HaChaim 72; Mishnah Berurah 24).
12. During the morning prayers on the day before Rosh Hashanah we do not recite Vidui, nor do we say Tachanun or Nefilat Apayim. And we do not say Lamnatzeach (Psalm 20) and Tziduk HaDin.
13. On the day before Rosh Hashanah it is customary to have one's vows annulled after the Selichot, before the morning prayers, for whoever does not keep his vows is considered excommunicated, and the prayers of one who is excommunicated are not received for forty days (see Kaf HaChaim 581:12, 19).
14. One should be careful about his afternoon Mincha prayer on the day before Rosh Hashanah, for this is the last prayer of the year. One who is fasting the entire day should say Anenu; however, Anenu is not said in the prayer leader's repetition (see Ben Ish Chai, Nitzavim 2).
15. Some have the custom to fast on the day before Rosh Hashanah (see Shulchan Arukh 581:2). One who commits himself to fasting in the Mincha prayer on the afternoon before the fast itself, fasts until the onset of the festival and says Anenu in his silent reading of the Mincha prayer before the festival. Some are accustomed to fasting just until after the Mincha prayer. At that time, they have a taste of something in order not to enter the festival in a state of fasting. One who follows this latter practice does not say Anenu in Mincha. If a person suspects that the fast will harm him, or cause him to be weak on Rosh Hashanah evening, he should not fast (Later Authorities; see Kaf HaChaim 61).
16. One who wishes to fast on the day before Rosh Hashanah and does not sleep on the night before the fast, can eat until dawn (Amud HaShachar). If one slept and awoke before dawn, he may not eat a thing, even before Selichot. However, it has become the custom to drink coffee and other beverages before dawn (Kaf HaChaim 581:69, 70; Ben Ish Chai, Nitzavim 1).
17. If one who fasts has to participate in a Seudat Mitzvah (meal that accompanies a religious celebration), he may eat on that day and fast on another day instead. Likewise, if one knows beforehand that he will be taking part in such a meal, he may fast a day earlier. In such a case, one should commit himself to fasting in the Mincha afternoon prayer on the day before the fast.
18. Even if ten Jews who are fasting the entire day pray together, the prayer leader is not permitted to say Anenu in his repetition, and they may not take out a Torah scroll to read the "Vayachel" portion (Kaf HaChaim 568:42).
19. If a person who fasts every year on the day before Rosh Hashanah cannot fast one year, he should do an annulment of vows for this custom (see Shulchan Arukh, Yoreh Deah 214:1, and see Pitchei Teshuva ad loc. 1).