http://www.berachot.org/halacha/24_birkathachammah.html
4. Birkat Hachammah
The Gemara berachot 59b states:
תנו רבנן: הרואה חמה בתקופתה לבנה בגבורתה וכוכבים במסילותם ומזלות כסדרן, אומר ברוך עושה בראשית. ואימת הוי? - אמר אביי: כל עשרים ושמונה שנין, והדר מחזור ונפלה תקופת ניסן בשבתאי באורתא דתלת נגהי ארבע
“Our Rabbis taught: On seeing the sun at its tekufah, the moon at its strength, the stars in their paths, and the Zodiacal constellations in their order, one says Blessed be the Maker of creation. And when does this happen? Abaye said: Every 28 years when the cycle begins again and the Nisan equinox falls into Saturn on the evening of Tuesday, going into Wednesday.”
Abaye’s explanation is based on Shmuel’s calculation that each Tekufah (season) is 91 days and 7.5 hours long. Therefore, 4 seasons, or one solar year, would be 365.25 days (or 365 days and 6 hours). Additionally Abaye’s explanation assumes Rav Yehoshua’s position regarding the creation of the world:
(There is a famous dispute in Gemara Rosh Hashana 10-11 when the world was created: Rav Yehoshua says it was Nissan while Rav Eliezer says it was in Tishrei – the Gemara on 12a concludes that we hold like Rav Yehoshua for calculating the tekufot and Rav Eliezer for numerating the years).
The sun and the moon were first set into place on the evening of the fourth day of creation, or in our terms – Tuesday evening at 6:00 PM. That was the first Tekufat Nissan. One solar year later was the second Tekufat Nissan. Since a solar year, according to Shmuel, is 365.25 days – and since 365 comprises 52 weeks plus a remainder of 1.25 days – the second tekufat Nissan occurred 1.25 days later in the week than the first tekufat Nissan. Thus, the second tekufat Nissan was 1 day and 6 hours (or 30 hours) after Tuesday 6:00 PM, which would be midnight 12:00 AM between Wednesday and Thursday.
If you were to keep adding 30 hours to the first Tekufat Nissan (Tuesday 6:00 PM…Wednesday 12:00 AM…Friday 6:00 AM…Saturday 12:00PM…Sunday 6:00 PM…Monday 12:00 AM…Wednesday 6:00 AM…ect…) it would take 28 years until the Tekufat Nissan returned to Tuesday 6:00 PM.
If you notice the pattern, every 4 years the tekufah returns to the same time of the day (6:00 PM). Also, since 1.25 times 4 is 5 - Every set of 4 years will start 5 days later in the week. So if the first set of 4 was on a Tuesday, the second set of 4 would be on a Sunday. The third will be on a Friday, the fourth on a Wednesday, the fifth on a Monday, the sixth on Saturday, the seventh on Thursday and finally at the very beginning of the eighth cycle it will be back to Tuesday. So it takes 7 full sets of 4 to return to the same day of the week, hence it takes 28 years.
In a sense this is the sun’s anniversary which we celebrate by making our special birkat haChammah on. Therefore this April 8, 2009 in the morning we will be saying Birkat Hachammah