A ritual to commemorate the creation of the sun.
According to rabbinic calculations, once every 28 years, on the same day of the week and at the same hour of the day, the sun returns to the position where it was when it was first created.
At this auspicious moment, the rabbis legislated, it is a mitzvah to take part in a Birkat Hahammah (Blessing of the Sun) ceremony.
The Talmud states: "He who sees the sun at its tekufah (season) should say, 'Blessed be the Maker of Creation' (barukh oseh bereishit)." Abaye explains that the tekufah of the sun occurs "Every 28 years when the cycle is repeated and Tekufat Nisan falls in Saturn on the evening of Tuesday, going into Wednesday." (Berakhot 59b)
Tekufat Nisan refers to the vernal equinox, which is the exact time when spring begins. At the vernal equinox, the night and day are exactly the same length, with night beginning at 6:00 PM and day beginning at 6:00 AM.
According to Genesis, God created the sun on the fourth day. Abaye's position relies on the assumption that that day was in the Hebrew month of Nisan (more accurately, at the end of Adar), and that it was the day of the vernal equinox. Since the Jewish day begins in the evening, the fourth day began at 6:00 PM on Tuesday. Abaye assumed that God wasted no time--so it was precisely at that moment that God created the sun.
But why does this recur only every 28 years? Dr. Daniel Lasker, of the Goldstein-Goren Department of Jewish Thought at Ben-Gurion University, explains the math on the Seforim blog:
The assumption shared by many in the ancient world was that the solar year was exactly 365 1/4 days long. On the basis of this calculation, the tekufot (equinoxes and solstices) progress each year by one day and six hours (365 1/4 days is 52 weeks, one day, and six hours). If the first vernal equinox was on Tuesday night at 6:00 PM, the next one was Wednesday night at midnight; the next one was Friday morning at 6:00 AM; then Saturday at noon; Sunday night at 6:00 PM, and so forth.
By this logic, it took 28 years after the year of creation for the vernal equinox to fall again on Tuesday night at 6:00 PM. And then it continued to recur every 28 years. On the 14th of Nisan, 5769 (the evening of April 7, 2009) the vernal equinox will be at 6:00 PM on Tuesday for the 207th time.
Lasker points out that Abaye's ruling is based on some problematic assumptions: "The Bible gives no indication that the sun was created on the equinox (either vernal or autumnal), or that it was created at 6:00 PM on Tuesday night (after all, before the creation of the sun, there was no 6:00 PM). But most significantly of all, the year is not exactly 365 1/4 days long."
Based on all we know today about astronomy and the natural world, Lasker explains, "the Jewish world is blessing the sun as it returns to its original time at the vernal equinox on a date which has nothing to do with the true vernal equinox (which is this year on March 20, 11:44 UTC (Coordinated Universal Time)."