The Birkath Hahamah, the blessing on the sun, is recited every 28 years on a Wednesday morning, because the Sun was created on a Wednesday.
The season (Tequfa) of the sun starts on a Tuesday night, which is the beginning of Wednesday according to the Jewish calendar. When a person sees the sun in its season every 28 years, in the morning, he says the Berakha (blessing) of "Barukh... 'Oseh Ma'aseh Bereshith" (the One who makes the act of Creation).
What is the reason why we recite this blessing at this time on a Wednesday? The answer is that it is because the sun, at that time, is in the self same position that it was in at the time of Creation.
(See Ben Ish Hai, Shanah Rishona, Parashath 'Eqeb, Oth Yod Teth)
