This coming Wednesday, on the eve of Pessah, we perform a ritual that occurs only once every 28 years. The source for this commemorative event is a passage in the Talmud (B. Brachot 59b). It records a tradition about blessings over astronomical events: "One who sees the sun at the beginning of its cycle, the moon in its mightiness, the planets in their orbits or the signs of the zodiac in their order, should say - Blessed are You, God, our Lord, King of the universe, who makes the work of creation." Of the four astronomical events mentioned in the source, only one is commonly commemorated, that is "the sun at the beginning of its cycle." While the codifiers explain the other astronomical events - when the moon, the five planets visible to the naked eye and the constellations of the zodiac align in prescribed manners (see Maimonides, Laws of Blessings 10:18), it is not customary to recite the blessing over these events (Mishna Brura 229:9), perhaps because the calculations can only be made by a skilled astronomer and that task is beyond the ability of most people (Rabbi Ephraim Zalman Margoliot, 18th-19th centuries, Poland). The blessing over "the sun at the beginning of its cycle" is, however, recited. "The beginning of its cycle" means the time that the sun returns to the position in the heavens which it occupied when it was first created. The Talmud inquires: "When does it happen that the sun is at the beginning of its cycle?" The sage Abbaye responds: "Every 28 years the cycle begins again." Abbaye is referring to a major solar cycle; In truth the sun returns to its original position once a year, yet the blessing is prescribed for when the sun reaches its original position, on the same day and at the same hour of its original placement at the creation of the universe. The sun was created and placed in the heavens on the fourth day of creation (Genesis 1:14-19), thus the blessing is always recited on the fourth day of the week - Tuesday night-Wednesday. Elsewhere in the Talmud there is a disagreement as to whether the world was created at the beginning of spring or at the beginning of autumn (B. Rosh Hashana 11a). Our talmudic passage adopts the approach that the world was created at the beginning of spring, that is in the month of Nisan. Abbaye goes further, detailing the exact hour of the event and hence the ritual: "When Tekufat Nisan falls in the hour of Saturn, on the evening of Tuesday which is the night before Wednesday." Nisan is the month when spring begins and Tekufat Nisan refers to the vernal equinox, that is, the 24-hour period when the day and night are equally long. On this day sunset is at 6 p.m. and dawn is at 6 a.m.; there are 12 hours of darkness and 12 hours of daylight. The vernal equinox is an annual occurrence, yet for the blessing it must occur on a particular day of the week, at a particular time. That is the meaning of "in the hour of Saturn." Our sages referred to seven celestial bodies - Saturn, Jupiter, Mars, the sun, Venus, Mercury and the moon, whereby each body is said to "serve" for one hour starting at the beginning of the week in a continuous cycle. "The hour of Saturn" refers to the day of the week that begins with Saturn "serving," that is Tuesday evening. The reference to "Saturn serving" is an astrological phenomenon, not an astronomical calculation, since the orbit of Saturn around the sun is calculated at 29 years and 167 days. Thus the vernal equinox must fall at the beginning of the fourth day of the week, that is, at 6 p.m. on Tuesday. While the "hour of Saturn" happens every week on Tuesday evening, while the sun can be found in its original position once a year - these three events coincide once in 28 years; hence the blessing is recited once every 28 years. The sun, however, is not visible in all places at 6 p.m. on Tuesday. Thus the following morning - on Wednesday morning - we go outside to look at the sun and recite the blessing. Even if the sun is hidden by clouds, but its rays shine forth, the blessing may be said. While ideally we recite the blessing early in the morning, if it is so cloudy that the rays of the sun cannot be discerned, we wait until midday in the hope that we will still see the sun. If by midday the sun has still not appeared, we recite the blessing without God's name, and wait for 28 years until the next occurrence. It is customary that this rare ritual is done with a group of people, for it is considered a greater honor for the Almighty when mitzvot are performed en masse. This event is referred to as Birkat Hahama (the blessing of the sun) and although the Talmud prescribes a lone short blessing, a blessing that is also recited upon seeing other natural phenomena, in recent generations additional passages have been added to the ritual, presumably to add to the atmosphere and to acknowledge the rarity of the event. Twenty-eight years ago, on Wednesday, April 8, 1981, 4 Nisan 5741, as a young schoolboy in Melbourne, I remember that the entire school went outside and stood in front of the synagogue. Together we recited Birkat Hahama. Sure I did not know what an equinox was, but our teachers explained that the sun had returned to the very same point it had been when it was first created and we were gathered together to acknowledge that God had created the world. I am sure that in 28 years time on Wednesday, April 8, 2037, 23 Nisan 5797, when we and our children once again stand and recite Birkat Hahama, we will remember - some more vividly than others - where we stand this coming Wednesday. The writer is on the faculty of Pardes Institute of Jewish Studies and is a rabbi in Tzur Hadassah.