Interesting answer from
this post at Treehugger. "See, way back in 1845, Congress changed the voting laws in an apparent attempt to make things more convenient for voters. Since we were an agrarian society, many of these voters were farmers traveling from the country into the towns and cities to vote. Since, at that time, people believed God forbid traveling on Sunday, Tuesday was a good choice because it gave them Monday as a travel day. Wednesday was the day these farmers would sell their crops in the market, so Tuesday became election day. That's pretty much it."