General Election Day.
Elections for all federal elected officials are held on the Tuesday after the first Monday in November in even-numbered years (November 5, 2002 for the next mid-term congressional elections, and November 2, 2004 for the next presidential election); presidential elections are held in every year divisible by four. Congress selected this day in 1845 (5 Stat. 721); previously, states held elections on different days between September and November, a practice that sometimes led to multiple voting across state lines, and other fraudulent practices. By tradition, November was chosen because the harvest was in, and farmers were able to take the time needed to vote. Tuesday was selected because it gave a full day’s travel between Sunday, which was widely observed as a strict day of rest, and election day. Travel was also easier throughout the north during November, before winter had set in. ... In most rural areas, the only polling place was at the county seat, frequently a journey of many miles on foot or horseback.
I do seem to remember from history that some states did vote earlier in Lincoln's day and thus gave an indication how the voting was likely to go in the country as a whole. Either those were non-Presidential elections, or those states had been allowed to keep their earlier arrangements. The 1845 law may just have been a guideline suggested to get the states started if they chose to line up on the same day.