The Minhag to Recite Selichos Before Rosh Hashanah
The minhag of Sephardim is that they start reciting selichos from Rosh Chodesh Elul, and they continue all the way until Yom Kippur. This is in accordance with the Mechaber (581:1) who writes: "The custom is to get up early to recite selichos, from Rosh Chodesh Elul and on, all the way until Yom Kippur."
The minhag of Ashkenazim, however, is like the Rema who writes: "The minhag of Bnei Ashkenaz is not like this ... they get up early to recite selichos on the Sunday before Rosh Hashanah, and if Rosh Hashanah falls on Monday or Tuesday, then they start already from Sunday the week before."
Waking Up Early for Selichos
The Mechaber (581:1) writes: "The minhag is to wake up early to recite selichos and special supplications." The reason for this, writes the Levush, is based on a Medrash which says: During the night Hashem flies over eighteen worlds, and in the last three hours of the night, He flies over our world, and He is close to anyone who calls out to Him at that time.
The Shu”t Teshuvos V’Hanhagos (4:134) writes: Even if one recites selichos at chatzos [midnight], he should still make sure to get up early for Shacharis, as showing hastiness to serve Hashem in these days is a big tikkun [fixing]. He adds: Those who say selichos at midnight without having gone to sleep, or they get up at a normal time, miss out on this special opportunity. They bring from R' Aharon of Belz that even though he would normally be up the entire night, during the days of selichos he asked his gabbai to allow him to sleep a little, even just a few minutes, so that he could fulfill the halachah of getting up early for selichos.
The sefer Darkei Chaim VeSholam brings that the Minchas Elozor wouldn’t allow selichos to be recited after 7:00 AM. Seemingly, his reasoning was, that by 7:00 AM most people get up anyway even without selichos, therefore, saying selichos at such a late time can no longer be considered getting up early for selichos.
There needs to be at least four days of selichos
Therefore, if Rosh Hashanah falls towards the beginning of the week (like this year), we start selichos the week before to ensure there is at least four days. Selichos during the Aseres Yemei Teshuvah, have a special advantage compared to the selichos recited before Rosh Hashanah as the Aseres Yemei Teshuvah are a special time for teshuvah, like it says in Yeshayah (Perek 55): "Seek out Hashem when He can be found", which the Gemara in Rosh Hashanah (18a) tells us refers to the Ten Days between Rosh Hashanah and Yom Kippur. Therefore, when talking about selichos before Rosh Hashanah the Shulchan Aruch writes "there is a custom", since it is merely a custom, however, when talking about selichos during Aseres Yemei Teshuvah the Shulchan Aruch (602:1) writes it in a more obligatory manner and says: "In all the days between Rosh Hashanah and Yom Kippur one should increase in tefillah and in supplications."
Reasons for The Minhag
The Ran (Rosh Hashanah 27) writes that according to Rebbi Eliezer the world was created on the 25th of Elul, therefore, we wake up early to say selichos starting from the 25th of Elul, which is four days before Rosh Hashanah. Some are medayek from this Ran, that selichos should always start on the 25th of Elul, regardless of when Rosh Hashanah falls out.
An additional reason mentioned by the Ateres Zekeinim (s.k. 8, mentioned by Mishnah Berurah 581:6) is that we find by korbanos that they need "bikkur mumim". "Bikkur mumim" is a process where we check a korban from four days before it is shechted [slaughtered] to make sure that it doesn’t have any mumim [blemishes] which make it pasul [disqualified]. Therefore, we start reciting selichos from four days before Rosh Hashanah, as unlike all the Mo’adim where it says in the Torah "you shall offer up an olah", by Rosh Hashanah it says "you shall make an olah", which means that on Rosh Hashanah a person should make himself like a korban. Since on Rosh Hashanah we are like a korban, we check for spiritual mumim from four days before, in order that if there is any, we have time to fix them.
A further reason mentioned by the Levush (581:1, cited in Mishnah Berurah 581:6) is that there are those who have the minhag to fast during the period of Yomim Noraim for ten days corresponding to the Aseres Yemei Teshuvah. However, there are four days during the Aseres Yemei Teshuvah where one can’t fast, the two days of Rosh Hashanah, Shabbos Shuvah, and erev Yom Kippur, therefore, they make up for these four days with four days of selichos. However, when Rosh Hashanah falls on Monday or Tuesday, and there isn’t four days from motzei Shabbos until Rosh Hashanah, they start from Sunday the week before, "And in order that there is a fixed day to avoid confusion, they fixed to always start on Sunday."
