Once Every 28 Years!

What is Birkat Hachamah? An Overview of this Rare Blessing

G‑d made the two great lights: the greater light to rule the day, and the lesser light to rule the night . . . And it was evening and it was morning, a fourth day.—Genesis 1:16, 19 One who sees the sun at its turning point should say, "Blessed is He who reenacts the works of Creation." And when is this? Abaya said: every 28th year.—Talmud, Tractate Berachot 59b

see http://www.chabad.org/library/article_cdo/aid/817862/jewish/About.htm