It is out of date, written when Loma Linda still had Sunday delivery (date on it is June 2009).
According to this, the 1957 suspension of Saturday service lasted for exactly one Saturday, which explains why people mention a particular date (April 13) for this suspension, but no _end_ date -- that's because the suspension only applied that one day.
The LBJ suspension was partial but lasted a lot longer, and I would characterize it as yet another example of the 1960s screwing cities and favoring suburbs, since the only deliveries suspended were the ones done on foot.
"In May 1964 the Post Office Department ended Saturday delivery of Parcel Post in 6,091 cities where carriers made deliveries on foot – again, to save money. Delivery resumed in January 1966 after President Lyndon B. Johnson promised to seek increased funding from Congress. Johnson considered “a good, stable, dependable postal system . . . vital to the well-being of the nation’s economy.”7"
Footnote 5 addresses, indirectly, the Poor Rural People Really Must Have Saturday Delivery:
"Tri-weekly rural routes were established where mail volume was not sufficient for six-day delivery. The 1906 Annual Report of the Postmaster General (page 330) indicates that .6% of rural routes were tri-weekly. A postal survey in 1999 found that approximately .06% of rural customers received tri-weekly delivery."