According to Gary Wills (in his book Head and Heart: American Christianities) Sunday mail delivery was stopped in 1912 as one of the final acts of the Third Great Awakening.
If America was founded as a Christian Nation like the Christian Right so often claims, why was mail delivered on Sundays until 1912? That's right, the absence of mail on Sundays is actually a modern development. The so-called 'Christian' founders didn't have a problem with Sunday mail delivery.
When in 1810 Congress brought the national postal system into existence, it legislated for seven-day mail delivery without anyone initially raising the problem of Sabbath violation. By 1828, however, with national religious consciousness growing, local religious leaders began to complain that post offices, which doubled as gathering places in small towns, were diverting the faithful from attending church on Sunday. Committees were formed in both the North and the South to demand that mail on Sundays be stopped.
Consider this very carefully: Sunday mail service wasn’t the object of criticism because it caused work on Sundays, thus violating prohibitions against working on the Sabbath. Instead, it was the object of criticism because it meant that post offices were open on Sundays, and this created competition for churches holding Sunday religious services.
In other words, Christian churches didn’t like the very minor competition created by an open post office and wanted the government’s assistance in eliminating that competition. They wanted Sunday all to themselves — they didn’t want anything to divert people’s attention from their church services. Because they recognized that what they offered was so poor, however, they wanted government help in doing this.
We can find the exact same thing happening today, too.
What followed was the first major national discussion of the proper relationship between the federal government and religion since the ratification of the Constitution. Stating that “our Government is a civil and not a religious institution,” Kentucky Senator Richard M. Johnson took the lead in arguing that the law should not be changed. Johnson made the pragmatic argument that ending Sunday delivery would delay mail service on the other six days and slow the growth of the national economy.
But he also emphasized the principled claim that changing the law would require the government to take a stand on what day, Saturday or Sunday, was the Sabbath. Once the government began to “determine what are the laws of God,” Johnson warned, there would be no stopping the rise of religious oppression. What was more, worried Johnson, the fact that religious groups were acting in concert to end Sunday mail service presaged further political activity by “religious combinations.” Here was a political concern that Madison would have found familiar: religious diversity might cease to protect nonestablishment when different religious denominations could form alliances.
http://atheism.about.com/b/2006/02/19/sunday-mail-service-in-a-christian-nation-book-notes-divided-by-god.htmhttp://en.wikipedia.org/wiki/Third_Great_Awakening