The Rama (Rav Moshe Isserles of Cracow, 1530-1572) mentions the custom to give charity to the poor on Ereb Rosh Hashanah. This practice is based on the verse in the Book of Nehemia (8:10) in which the prophet instructed the people on Rosh Hashanah to enjoy fine foods and drinks, and to give food packages to the poor so they can enjoy the Yom Tob. Since we cannot give food packages on the Yom Tob, it is proper to give either food or money to the poor on Ereb Rosh Hashanah. This custom is mentioned also by Rav Haim Palachi (Turkey, 1788-1868) in Mo’ed Le’kol Hai (12:2).
Rav Haim Vital (1543-1620) had the practice of making "Pidyon Nefesh" – the "ransom of the soul" – on Ereb Rosh Hashanah. He would count 160 coins and put them in a safe location on Ereb Rosh Hashanah, and then declare the "Pidyon" on Ereb Yom Kippur. He would distribute the money to the poor on Ereb Sukkot. It is taught that those who follow this practice are guaranteed to remain alive through the coming year. Hacham Natan Ben-Sanior (contemporary) writes that as this money is given as a "ransom," it may not be counted towards one’s Ma’aser Kesafim (tithing of income). We observe this practice in our community, with members donating $100 for the Pidyon, and the money is distributed to the Rabbis of a local Kollel before Sukkot.
It is customary also to perform Hatarat Nedarim – the annulment of vows – on Ereb Rosh Hashanah.
There is a special Misva for women to bake bread on Ereb Rosh Hashanah and perform "Hafrashat Halla" – separating a portion of dough. The reason for this custom is because Adam and Hava were created on Rosh Hashanah, and on that same day, Hava ate from the forbidden tree and shared the fruit with Adam. Adam is called "Halato Shel Olam" – "the Halla of the world," and thus, as Hava caused Adam’s downfall, women correct this mistake by separating Halla on Ereb Rosh Hashanah, the anniversary of that sin.
One should try to recite Minha on Ereb Rosh Hashanah – the last prayer of the year – with extra special concentration and intensity. Tradition teaches us that prayers which are recited without Kavana (concentration) do not have "wings" with which to rise to the heavens and achieve their intended purpose. Therefore, all the prayers which we’ve recited over the course of the year without proper concentration are pending, in a sense, unable to rise to the heavens. These "pending" prayers can reach the heavens either through the prayers of a righteous Sadik who is connected to the individual who recited these prayers, or by the person himself, if he prays again with special feeling and concentration. It is therefore worthwhile when reciting the last prayer of the year to pray with special concentration, which will have the effect of elevating all the prayers which were recited during the year without proper concentration. This point is made by the Ben Ish Hai, in Parashat Nisavim (2).
One should make every effort to immerse in the Mikveh on Ereb Rosh Hashanah. The Ben Ish Hai taught that one when immerses in the water the first time, he should have in mind simply that he immerses for the purpose of purity. The second time, he should have in mind that he immerses to repair the damage caused through his anger. The third immersion is to "sweeten" harsh judgments. The fourth and fifth immersions are to divest ourselves of our weekday soul and to receive the special soul of Yom Tob. During the final immersion one should also have in mind "Tahel Shana U’birchoteha" – that the new year should bring new blessings.
It is also worthwhile to recite Tehillim on Ereb Rosh Hashanah.
By observing all these customs on Ereb Rosh Hashanah, we involve ourselves in Teshuba (repentance), Tefila (prayer) and Sedaka (charity), which, as we know, have the effect of eliminating harsh decrees, so we will be worthy of a year of blessing, Amen.
Summary: There are several meaningful customs that should be observed on Ereb Rosh Hashanah:
1) Giving charity to the poor;
2) Hatarat Nedarim;
3) Baking and separating "Halla";
4) Immersing in the Mikveh;
5) Reciting Tehillim.
In addition, it is proper to make a special effort to recite Minha on Ereb Rosh Hashanah – the final prayer of the year – with special concentration and intensity.