It is interesting to note that Rav Saadia Gaon counts among the taryag (613) mitzvos the convocation ceremony that happened at Har Grizim and Har Eival on the day that the Jewish nation crossed the Jordan. The Torah in Parshas Ki Tavo (Ch. 27) details the exact manner that the new immigrants to the land were to gather - six tribes on Har Grizim and six tribes on Har Eival. The Kohanim and Levi'im stood in between the two mountains and first pronounced blessings to the nation if they keep the Torah, and then curses if they do not.
Most commentaries do not count this happening as one of the 613 mitzvos, as it was an experience that occurred but once. By definition mitzvos are eternal, thus the placing of the blood on the door post in Egypt is not counted as a mitzvah. Rav Yeruchum Perlow explains that this is the mitzvah of arvus – collective responsibility that the Jewish nation entered into at this time. The mitvzah is not to regroup at the mountain but to practice and live by "kol Yisroel araivim zeh lazeh" (all Israel is responsible for one another.)
This is the source and foundation for the important halachic principle of yatzah-motzi. Under normal circumstances, once an individual has fulfilled a particular mitzvah he is not given a second chance to fulfill that mitzvah again. Case in point: one who has recited kiddush Friday night cannot recite the kiddush again, claiming for example that he or she lacked proper kavanah( concentration) the first time. Yet, the same individual, should he go for a walk that night and encounter a Jew who has not yet recited/heard kiddush, is obligated to recite kiddush for him, as all Israel is responsible one to another.
The meaning of the word "areiv" in the above context is that of a co-signer. As in a loan, the co-signer assumes responsibility for payment if the borrower renegs on his obligation, similarly each Jew is responsible to see that his fellow Jew observes Torah and mitzvos. Your mitzvah of shofar, lulav, sukkah and T'fillin are incomplete if your neighbor has not yet fulfilled the mitzvah, and you can positively correct the situation.
The Noam Elimelech suggests another interpretation to the word areiv in light of the passage in Shir Hashirim (2:14) - "ki koleich areiv" - "for your voice is sweet" - kol Yisroel araivim zeh lazeh - all Israel sweetens one another. It is not coincidental that we read Ki Tavo and Nitzavim each year prior to Rosh Hashana. We are taught "tichleh shanah v'killoseha" = may the year with its curses and adverse circumstances come to a close. I'd like to suggest that the mitzvah of arvus is a critical one, especially at this time of the year.
Rosh Hashanah is a two day holiday even in Eretz Yisroel. The Talmud Rosh Hashanah (30b) gives an interesting halachik explanation. Rav Dessler (Michtav M'ELiyahu vol. 2) cites a fascinating Zohar (Parsha Pinchas) which offers the following philosophical approach: The first day of Rosh Hashanah is a dina kasha – rough/strict judgment, while the second day is that of dina rafya – soft judgment. Rav Desler then brings the explanation of the Arizal to the above. The first day of Rosh Hashanah each individual is judged as an individual, how they fared with their unique individual potential. Each person has character traits and understanding that is exclusively theirs. The Talmud (Berachos 58a) teaches that just as each person's facial features are unique, so is their perception and intelligence. How one either elevated or used and abused their individual personal gifts and potential is judged on day one. All too often the result is that man is found to be lacking, hence dina kasha – difficult judgment.
The second day however, explains the Arizal, man is judged as part of the klal, his being a member of Klal Yisroel. Here there is greater opportunity for dina rafa – softer/kinder judgment as each individual contributes - areivim – sweetens and enhances the community. Your praying with a minyan in synagogue as opposed to praying alone at home is not only a higher quality personal mitzvah but each participant contributes to "brov am hadras melech." The quality not just quantity of one minyan is enhanced by each individual . Moreover, note the text of the berachos of the Shemoneh Esreh- they are all couched in the plural. We do not only pray for our health, wealth, and personal redemption but include the rest of Klal Yisroel as well.
As each person is singular – unique – so too is their prayer on behalf of the Jewish nation. Thus, each one is needed. Therefore a most important way to prepare for Rosh Hashanah is to focus your prayers properly on behalf of the community enabling one to merit a positive judgement on Rosh Hashanah.
Finally, before Rosh Hashana pause and reflect how you can sweeten the lives of those around you. The smile that you bring to the faces of the senior citizens you visit, either alone or with your children, your telephone call to the shut-in inquiring of their well-being, Your greeting a newcomer to shul with a warm welcome as opposed to ignoring him. These are but a few practical methods of implementing the lesson of the Noam Elimelech. Make a conscious effort to sweeten the next one's day as there is no telling how it will impact on your life.