Found it, Congress didn't mandate Saturday delivery until 1981. Bigger areas were probably getting 6 day delivery before the area I grew up in since it wasn't required.
from---->
http://digital.library.unt.edu/ark:/..._2009Jun09.pdf
Since 1863, the U.S. Postal Service (USPS) has been delivering mail to homes and businesses in
cities. In 1896, USPS extended delivery services to rural locations. Although USPS has
historically delivered mail to most locations six days per week, it was not until 1981 that
Congress statutorily required six-day delivery. Today, USPS delivers mail and parcels to more
than 149 million homes, businesses, government offices, and post office boxes six days per week.
Throughout the Service’s history, however, there have been discussions about reducing the
number of delivery days to conserve fuel and reduce costs.