It is written (Hoshei'a 6:2), השלישי ביום מימיים יחיינו לפניו ונחיה יקמנו. The Vilna Gaon zy'a says that this pasuk refers to the akeidah. The akeidah took three days. The first two days were days of preparation; Avraham saddled his donkey, chopped wood, and traveled to Har HaMoriah for the akeidah. The third day was when the actual akeidah took place.
Chazal tell us that parnassah comes in the merit of the akeidah. The Vilna Gaon clarifies that parnassah is earned in the merit of the first two days, the days of preparation. Therefore, it states, מימיים יחיינו, our life, and our parnassah come from the first two days. לפניו ונחיה יקמנו השלישי ביום, the reward of the third day is reserved and will be given to Avraham in Olam HaBa.
This is because Chazal teach us, "There is no reward in this world for the mitzvos". The reward is reserved for Olam HaBa. However, we are rewarded for preparing for the mitzvos.
Now are the days of preparation, for which we will earn a great reward also, in this world.
The Beis Avraham (Netzavim) writes, "Elul can be compared to erev Shabbos and Tishrei to Shabbos. The sefarim say that the bounty that comes from Shabbos to the weekdays comes specifically from Tosfos Shabbos (Friday afternoon and motzei Shabbos, the weekday hours that one adds on to the Shabbos). This is because the kedushah of Shabbos is very great [and it is too high to bestow blessings to the weekdays. Therefore, the brachos which come from Shabbos come specifically from tosfos Shabbos.] The same can be said about the bounty and brachos that come to us the entire year. They come specifically from the "tosfos teshuvah", the teshuvah and good deeds we do in Elul, before Tishrei arrives."
If most of Elul has passed, and one didn't begin doing teshuvah and good deeds, he should begin now, during the final days of the year. This will bring us bounty and blessings for the upcoming year.
The Gemara (Rosh Hashanah 2a) states, "One day of the year is considered like a year." The Gemara says that this halachah is related to kings. If a king became king, even just one day before the end of the year, it is considered that he was king the entire year. The following day, when the new year begins, it will be written in documents "the second year to King Ploni." It is the king's second year, because the first year was completed in one day. The Chidushei HaRim zt'l says that the same can be said about a person who accepts the yoke of Hashem's kingship on the final day of the year. It will be considered like he had made Hashem the King for the entire year. Even during the final moments of the day, before Rosh Hashanah, he can detach himself from the yetzer hara and accept Hashem as King. As the Rambam (Hilchos Teshuvah 2:2) states, "What is teshuvah? A person should leave his sin, and remove it from his thoughts, and decide in his heart that he won't commit this aveirah anymore." One can have this thought a moment before Rosh Hashanah as well, and it will be effective.
Once, on erev Rosh Hashanah, just a few minutes before Rosh Hashanah arrived, the Chiddushei HaRim told a chassid that it isn't too late to do teshuvah. It is the final day of the year, an opportune time for doing teshuvah, and as Chazal say, ככולו היום מקצת, part of the day is like the entire day. So, in these final moments, he can fix the day, and he can fix the entire year. All this can be achieved with sincere teshuvah.
In the piyut דין עורך ל-לא that we say on Rosh Hashanah and Yom Kippur, there is a phrase, דין ביום נסתרות לצופה, "To Hashem Who sees the concealed on the day of judgment." Divrei Shmuel zt'l explains that people prepare for Rosh Hashanah, hoping that they will be able to daven properly, but then, when Rosh Hashanah arrives, they might find that their hearts are closed, and they are unable to daven. They see others, who didn't prepare for the yom tov, and they seem to daven with hislahavus. The piyut tells us דין ביום נסתרות לצופה, Hashem sees the concealed. Hashem sees the brokenness of your heart, and Hashem knows how much you prepared for Rosh Hashanah, and these are your merits.
As the Tzemach Tzedek said to a chassid who complained that he prepares for tefillah, but when he davens, he finds himself unable to daven. The Tzemech Tzedek told him, "Why should you care? You already davened during your preparations." He was explaining that preparation for tefillah is part of the tefillah. So, regardless of whether he was able to daven with kavanah or not, he has tefillah, because of his preparations.
Sweetness of Avodas Hashem
In the zemiros before Kiddush (בשבחין אזמר) it states, ודובשין במתיקין לשמיה דיתענג עמיה על דתשרי קמיה רעווא יהא. The Beis Avraham (תשרי ה"ד נצבים) explains it as follows: רעווא יהא, it should be Hashem's will, דתשרי קמיה, that before Tishrei arrives, עמיה על, the Jewish nation should be aroused to teshuvah and to perform good deeds. לשמיה דיתענג ודובשין במתיקין, and they should feel the sweetness of avodas Hashem, and then they will be blessed with children, life, and parnassah.
