We observant Jews count the Omer - 50 days from Pessah to Shavuot - because the Torah commands us to. Every night we've been reciting the number of days and weeks that have passed and look forward to the coming festival, rightly called Shavuot, or Weeks. In Greek it is Pentecost - the counting of 50. This counting of 50 days between festivals seems to have been an ancient custom among farmers. Many of the festivals are two months apart. If the farmer celebrated seven days of harvest festival, added an extra day, as the eighth day of Shemini Atzeret is added to Succot, and if he then counts 50 days, he is on to the eve of the next festival. This basic count of 59 days, or just two full months, would have applied between Pessah and Shavuot if we counted as the pre-rabbinic Ethiopians did, taking "from the morrow of the Sabbath" (Leviticus 23:15) to be the day after the whole festival, thus bringing Shavuot to 15th of Sivan. It was the Pharisees who changed all that so as to bring Pentecost forward to coincide with their date for the Theophany at Mt. Sinai. THE JEWISH calendar has been a lunar one, confirmed by the sighting of the new moon, at least from the time of the little tablet, the so-called Gezer Calendar excavated in 1908, considered to be of the 11th century BCE, which counted the agricultural year in consecutive months, four of them in pairs. In matters of time, the month is a natural phenomenon, as are the day and the solar year, although the three are difficult to correlate. The month is actually 29.530588 days long and the solar year 365.2422 days, which makes calculation difficult. In the early Roman Empire the year was still counted as a lunar one and correlation with the seasons was chaotic and unwieldy. It was so until the time of the Emperor Julius Caesar, who was advised by astronomers to scrap the lunar count and fix a solar year of 365 and 1/4 days. That fraction of a day was not practical so it was decreed to have three years of 365 days, and one of 366 days every four years. Known as the Julian Calendar, it had the desired effect of regulating the days and months with the seasons. The new reckoning was introduced in 45 BCE and was expected to be correct for time everlasting. But it was not to be. THE CALENDAR year was more than 11 minutes too long, which meant an increase of one day in about 130 years. Thus by medieval times, after a period of say, 1,000 years, the calendar year was already seven days in advance of the sun. By the time reform came, under Pope Gregory XIII, the calendar was 10 days ahead of the solar system, and consequently in March 1582, the new Gregorian Calendar took 10 days out of the month of October of that year. It was not an easy reform to swallow, and Great Britain was the last major country to adopt it, not doing so until nearly 200 years later in 1752. It was then that popular riots called for "the return of our 11 days," that led to the Treasury being forced to move the tax year 11 days forward from the quarter-day of March 25 to April 5. As the Julian year had been too long by about three days in 400 years, the Gregorian Calendar solved the problem by declaring that the leap day should be ignored at every century year and only used in those centuries divisible by 400, as is still the practice today. SO FAR, so good, but how does that impact on the Hebrew calendar? We are very meticulous in our counting, as for instance in counting the Omer between Pessah and Shavuot; and, after all, we did invent the seven-day week. It is now adopted all over the world, while the 10-day week of the ancient Egyptians and the five-day week of the more recent French Revolution never took hold. Our calendar has been a very ingenious one, having solved the problem of relating the lunar counting to the solar reality, and ensuring that festivals did not fall on unsuitable days of the week, like Yom Kippur on a Friday or Pessah on a Monday. But its very ingenuity should not lead us into thinking it infallible. When confirming the calendar by sighting of the moon became impractical, tradition has it that a fixed calendar was set down by the Patriarch Hillel II in 358 or 359 CE. It is not certain that this was so as, for instance, Maimonides does not mention it, though he says that monthly sightings did cease some time before the end of the Babylonian Talmud. Although some flexibility may have continued, it is clear that by the time of the Geonim in the ninth century a fixed calendar was being adhered to. That calendar, like all previous ones, divided the year into four tekufot or seasons, which conformed to the solstices and equinoxes of the earth around the sun. The first division was the tekufa of Nissan, or the spring season. It was in that season that the festival of Pessah had to fall, as the Torah tells us to "Observe the month of Aviv (Spring) and make Passover" (Deuteronomy 16:1), so the calculation of the four seasons was of paramount importance. ALTHOUGH we do not know when the fixed calendar started, whether in the fourth century or not, it is our guiding light today and 26 years ago it told us that the "Blessing of the Sun" was to be on April 8, 1981. It is a ceremony held every 28 years, when the sun is considered to be in the same position as the one it held in the days of Creation, that is, on a Wednesday. If the year is held to be 365 days long, then the date of its "birth" - its birthday - falls on the same day of the week every 28 years. But when we celebrated that in 1981, the tekufa date was April 8, or 18 days after the true astronomical equinox of March 21. In other words, the Jewish calendar was 18 days out of sync with the heavenly facts. How did this happen? Quite simply because the original calculations, whether by Hillel II or others, were based on the Julian Calendar of their time and, as we have seen, the Julian Calendar would now also be about 15 days out of line with the sun. Does this really matter? As the tekufa of Nissan, or Spring, continues to progress ahead of the sun, by more than 11 hours a year, we shall find that in time it will approach nearer and nearer to the astronomical summer, and the festival of Pessah will no longer fall in our tekufa of Nissan. It can be calculated that this will occur in about 640 years, and then we will be in breach of the Torah law. The festival may still be in the spring, but the tekufa of Spring will be in the summer. SO GREAT BRITAIN was not the last country to adopt the Gregorian correction of the Julian calculations. The Hebrew calendar has not yet recognized the discrepancy but it is something we should consider very carefully and quite soon, for the next "Blessing of the Sun" will occur in two yearsâ€š time, in April 2009, so it would be sensible to reform the calendar before then, so that our prayers will be in line with God's Creation, and not 19 days ahead of it. As in England, that change may well cause riots in the streets, this time in Jerusalem and Bnei Brak - but is it sensible to let this time bomb go on ticking any longer? The writer is a Fellow of the Albright Institute of Archaeological Research, Jerusalem.