The night before Erev Rosh Hashanah:
The Rebbe Rayatz stated: “The night before Erev Rosh Hashanah was considered the Wacht Nacht of the High Holidays. I do not remember that people went to sleep at a normal hour. Each person began his preparations for Erev R”H which introduces the days of judgment”.
Tehillim, Tefilah, Teshuvah:
One is to spend this day learning Torah, doing Mitzvos, and doing Teshuvah. One is to increase in saying Tehillim throughout the day, and repent in order so his prayers of Rosh Hashanah are clean and pure.
New Hiddur Mitzvah:
It is customary on Erev Rosh Hashanah to accept upon oneself a Mitzvah which one will keep more carefully. One is to accept the new Hiddur with saying “Bli Neder” in order to avoid it becoming a vow.
It is proper to begin doing repentance of sins between man and his fellow, which includes asking for forgiveness, prior to Rosh Hashanah. One should not delay doing so until Erev Yom Kippur.
The Peri Chadash would distribute Mishloach Manos on Erev Rosh Hashanah so the poor are able to eat good foods for the Yom Tov meal. It is likewise customary on every Erev Yom Tov to send all the necessary Yom Tov foods to the poor of one’s city. Those that are accustomed to give charity every single day of the week are to give charity on Erev Rosh Hashanah also on behalf of the two days of Rosh Hashanah.
Spending time with one’s spouse:
On Erev Rosh Hashanah, close to the entrance of the Holiday, the Rabbeim would spend some time speaking with their wives. The purpose of publicizing this matter is to teach a lesson to each person in the importance of Shalom Bayis, as despite the greatness of the day and the careful use of time by the Rabbeim especially on this day; nonetheless they made time to spend with their spouse.
 Sefer Hasichos 1940 p. 10
 This refers to the night before the Bris in which we guard the child from evil forces.
 Kitzur Shulchan Aruch 128/14
 Sefer Hasichos 1944 p. 1 in name of Rebbe Rashab; The Rebbe Rayatz instituted for the Temimim that starting from the first day of Selichos one is to recite the entire Tehillim every morning prior to daybreak. [Hakeria Vehakedosha 1941; Otzer Minhagei Chabad 75]
 Sefer Haminhagim p. 117 [English]; Likkutei Sichos 2/386; Igros Kodesh 7/204; Sefer Hamamarim 1/132; Sefer Hamamarim 1938 p. 24 and 30; Shemuos Vesippurim 1/94; Sefer Hamamarim 1907 p. 77; Hisvadyos 1990 1/17; See Otzer Minhagei Chabad p. 153-154; Shulchan Menachem 3/113; Chida in Machazik Bracha 603/2 [brought in Kaf Hachaim 603/12] “I witnessed men of stature that seclude themselves by the days of repentance and jot down on paper the new resolutions they take upon themselves.”
 This was the custom of the Rebbe Rashab as recalled by his son the Rebbe Rayatz in the sources ibid
 Sefer Haminhagim ibid; However see other sources quoted above that mention the Hiddur was accepted on Rosh Hashanah itself.
 In many of the sources above it states “One is to accept a Hiddur Mitzvah in positive commands, negative commands and good deeds.” [Likkutei Sichos 2/386; Shemuos Vesippurim ibid]
 Hisvadyos 1983 1/34 [brought in Shulchan Menachem 3/113; Shaareiy Halacha Uminhag 5/69]; Machazik Bracha 603/2; Kaf Hachaim 603/12
 Kaf Hachaim 581/59; Kitzur SHU”A 128/14
 Kaf Hachaim 581/101
 This is in play of the verse in Nechemia [8/9] “And they ate fatty foods and drank sweet beverages, and sent presents to those that don’t have.” This verse was said in reference to Rosh Hashanah.
 Alef Hamagen 581/120
 See Likkutei Sichos 20/450 regarding Shabbos Chanukah and so is written in various letters of the Rebbe; Shaareiy Tzedaka p. 142
 Hisvadyos Yud Shvat 1952 [Vol. 20 p. 72]; Sichas Kodesh 1975 1/4; 2nd day Rosh Hashanah 1988; Otzer Minhagei Chabad 72 [p. 56]