Minhagim and Customs of the Yamim Noraim (1)
Selichos. Most congregations begin to recite Selichos this מוצאי שבת, when we recite, time after time, י"ג מדות הרחמים (13 attributes of Divine Mercy) in order to activate Hashem’s mercy so that we may merit blessings for the new year. In the introductory phrase, before one recites the י"ג מדות, we recite the posuk: "ויקרא בשם ה'" - “He called out in the Name of Hashem.” (1) One should pause between the words "בשם" and "ה'" because the actual translation is: “He called out in the holy Name (to teach Moshe Rabbeinu the proper order of prayer. And Who was the One who called out?) - Hashem.” This is how the Mishna Berura (2) in the name of Avudraham (a later Rishon, talmid of the Tur) (3) explains the posuk. (It is proper to note that both Targum Onkelus and Rashi explain the posuk differently, as follows: “He (Moshe) called out in the Name of Hashem.” According to this, the words "בשם" and "ה'" are to be said together, without a pause. However, we follow the first way as the Mishna Berura rules.
With a Minyan. These are words of extreme holiness and can only be said with a minyan of ten people, similar to Kaddish and Kedusha. Therefore, if one is behind when the minyan gets to the י"ג מדות, he should pause where he’s up to and say the paragraph "א-ל ארך אפים" or "א-ל מלך יושב" with the minyan, then the י"ג מדות and one of the requests right after them, and then go back to where he was up to. If they are finishing reciting Selichos while he is still in the middle, he may finish it off alone, because since he started with a minyan, it is like he said the whole thing with a minyan (4).
Eruv Tavshilin: Understanding the Mitzvah. Over the next few weeks, the mitzvah of Eruv Tavshilin will be applicable on three occasions: Erev Rosh Hashanah, Erev Sukkos and Erev Shemini Atzeres. Eruv Tavshilin allows for Shabbos preparations when they are done early enough to derive benefit from them on Friday, e.g. putting up food early (5). Preparations may only be done on Friday, not Thursday (6). Also, the Eruv food should not be eaten (or become spoiled) until one has finished all of his Shabbos preparations (7), including "נר שבת". It is highly advisable to mark and refrigerate the cooked Eruv item.
(1) שמות לד:ה (2) תקפא:ד (3) דף פו. (4) כף החיים קלא:כד (5) משנה ברורה תקכז:ב (6) או"ח תקכז:יג (7) שו"ע הרב שם:א