Our Sages ordained that the Torah reading on Shavu’ot be about the revelation at Mount Sinai and the giving of the Ten Commandments. The haftara is from the beginning of Yeḥezkel, where the heavenly chariot is described (Megilla 31a; SA 494:1). As on all festivals, there are five aliyot (Megilla 21a). The original rule was to take out only one Torah scroll from the ark, and the maftir was one of the five people called up to read from it. However, the Ge’onim record the custom of taking out two Torah scrolls. Five people read from the first scroll, as described in the Mishna, and then maftir is read from the second scroll. The maftir is from Bamidbar and describes the festival offerings. According to the Gemara (Megilla 31a), the rationale behind this is that “God said: ‘For when the Temple no longer exists, I established [the texts about] the sacrificial order. Whenever the Jews read them, I consider it as if they offered the sacrifices, and forgive all their sins’” (Rosh; Ran; Beit Yosef 488:3; see above 2:8 n. 8).
Our Sages tell us that the Torah reading on Shavu’ot describing Matan Torah is especially important. “God said to the Jews: ‘My children, read this portion every year, and I will consider it as if you are standing before Sinai and receiving the Torah’” (Pesikta De-Rav Kahana §12).
On account of this, the custom on Shavu’ot is to read the Ten Commandments with the festive (“upper”) cantillation (ta’am elyon). When reading with the usual cantillation, one pauses at the end of each verse; when reading with the festive cantillation, one pauses at the end of each commandment. For example, the commandment to remember the Sabbath day extends over four verses, but it is read as one long verse with the festive cantillation. Likewise, there is a single verse that contains four commandments: “Do not murder,” “Do not commit adultery,” “Do not steal,” and “Do not bear false witness.” Normally they are read as one verse, but in the festive cantillation they are read as if they are four separate verses. This different division of verses affects the cantillation of so many words that a slightly different tune was established for reading according to this division (BHL 494:3). All agree that on Shavu’ot, the Ten Commandments are read with the ta’am elyon. Although there used to be various customs as to how to read them during the year in the normal cycle of Torah readings, nowadays at all public Torah readings it is standard to read the Ten Commandments with the festive cantillation.
Many stand during the reading of the Ten Commandments, as a remembrance of the awesome and magnificent event it describes. Listening to a public reading of the Ten Commandments is considered to be greeting the Divine Presence. The custom of standing is first mentioned in the time of the Rishonim about 800 years ago, but has become widespread only in the last 200 years or so. All Ashkenazim and some Sephardim follow this custom. Some object to it, because the Sages of the Gemara chose not to require the daily recitation of the Ten Commandments together with the Shema. They felt that this way, people would be less likely to be led astray by the heretical claim that it is sufficient to keep the Ten Commandments alone and disregard the rest of the mitzvot (Berakhot 12a). For the same reason, some object to standing for the Ten Commandments. Nevertheless, most poskim are not concerned about this. After all, the people stood at Sinai. They feel that the Sages were concerned only that the daily recitation of the Ten Commandments might lead people astray. Additionally, nowadays it is unusual to find heretics who argue that the Ten Commandments are the only obligatory mitzvot.