aussieboy-
A solar year is approximately 365.25 days, so every fourth year the fraction will add up to an extra day (which is why we have leap years). Therefore in the beginning of the fifth year the sun will be in the same location as it was in the beginning of the first year.
(Let’s assume that the beginning of the first year was exactly at sunrise, the beginning of the second year will be at midday, the beginning of the third year will be at sunset, the beginning of the fourth year will be at midnight and the beginning of the fifth year will be at sunrise…)
Since there are 7 days in a week and the LCM of 4 and 7 is 28, it will take 28 years until the sun is in the exact same place in the sky on the same day of the week at the beginning of the year.
After 2100 Birchas Hachama will no longer be on April 8th (it will still be on a Wednesday though (the first one being April 9th, 2121)) because 2100 is not a leap year as far as the Gregorian Calendar is concerned but is according to Halacha. (From 1800 to 1900 it was always on April 7th.)