Tomorrow is the start of the peak season for rabbis. They know that many Jews who have rarely set foot in synagogue during the past year will be putting in an appearance, and over the past few days they will have been polishing their sermons in the hope of making an impact while they have the chance.

For, over the next two days, Jews will be celebrating Rosh Hashanah, the religious New Year, and while it is both a joyful time to spend with family and friends, it is also a sombre one, too. The high spirited revelry associated with the New Year in some other cultures is noticeably absent. Rosh Hashanah marks the onset of the 10 Days of Repentance, culminating in the all-day fast of Yom Kippur, the Day of Atonement, which begins on Wednesday evening next week. It is a period of introspection, of spiritual stocktaking to review one’s moral failings and to resolve to live a better life.

As with the Sabbath, the religious holiday begins at dusk tonight and, as well as the wine and challah bread which customarily opens a festive meal, on this occasion a piece of apple is taken and dipped in honey to symbolise hope for a sweet year. On the second night, a new fruit is eaten with the meal, one not tasted for at least 30 days previously: pomegranates have often been a popular choice, because they are said to contain 613 seeds (the traditional number of the commandments).

To cope with the seasonal swell of worshippers, many synagogues have to hire additional halls or marquees to run overflow services. At four to five hours, the morning prayers over Rosh Hashanah are among the most demanding of the year, nearly twice the length of those on an ordinary Sabbath. (Yom Kippur is spent all day in prayer). The mantles of the Torah scroll and the curtain of the ark in which they are housed are changed to white, reflecting sobriety and purity.

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• Faith Online's guide to Rosh Hashanah and other festivals

Traditionally, Rosh Hashanah commemorates the Creation, the birthday of the world (a theme found in the Talmud, not the Bible). But it is another name for the festival which sets its mood, the Day of Judgment. According to tradition, all creatures undergo heavenly scrutiny, passing like sheep under the staff of a shepherd, in the image of one of the best-known prayers. “On Rosh Hashanah will be inscribed, and on Yom Kippur will be sealed, how many will pass from the earth and how many will be created,” the congregation sing.

This critical moment of the service, with its intimations of mortality, ends with the cry, “Repentance, Prayer and Charity mitigate the the harsh decree”. The Hebrew word for repentance actually means “return” and encapsulates the central, redemptive message of the whole season, its promise of the possibility of spiritual renewal. On the first afternoon of the festival, it is the custom to enact the casting away of one’s sins by going to a stream and scattering crumbs into the water.

For all the poetry of the prayers, it is not words, however, that are the definitive feature of Rosh Hashanah. The biblical command to observe the festival contains nothing about the New Year, it simply enjoins a day of “memorial of blowing” the shofar, the ram’s horn. The shofar is the elemental summons to spiritual re-awakening (sounded, according to the Torah, before the revelation at Sinai). When the shofar-blower sends its piercing, haunting shriek into the heart of the hushed congregation, even the most religiously enervated worshipper may feel some deep sense of connection stirring within.