Fat Tuesday is the designation generally given to the day just prior to the beginning of Lent. Lent is a 40 day time of fasting for many Christians and it always begins on a certain Wednesday prior to Easter. As the day before (always a Tuesday) is the last chance for revelry and also the last chance to use up perishables such as butter, lard, and other fats, that day has become known as "Mardi Gras" (French for Fat Tuesday).
The word "Tuesday", however, was derived from a wholly different religious tradition. Tyr or Tiw was the Norse god of war. When the Germanic Angles and Saxon's invaded England in the 500's they supplanted a culture that had been heavily influenced by Rome for several hundred years. The day, Tuesday, had already been named for the Roman god of war, Martius (notice in French, Italian, and Spanish - the word for Tuesday is still derivative of the Roman God - Mardi, Martedi, and Martes - respectively). When the Germanic tribes conquered England, they laid their own lexicon over that of the Romans so that the Norse god of war now supplanted the Roman god of war (after all the Norse war god was obviously more potent). Thus they called the day of the god of war tiwesdaeg.
Since 1845 in the United States, Tuesday has been the day that most voting takes place. This is because in the 1800s most polling places were only in the county seats. As counties could be large and travel slow, many people had to go to the county seat over-night. If elections were held on Monday, journeys would have to be taken on Sunday, this was taboo for many religions. Tuesday being the first convenient day after that, it was chosen.
Tuesday is considered unlucky by the Greeks, because Constantinople fell to the Turks on a Tuesday (29 May 1453). In the folk poem of weekday children, the days of the week poem goes, "Tuesday's Child is Full of Grace".
Monday | Wednesday