This is really confusing to me.
One is a day of bacchanalia and excess.
The other is a day of confession and penance.
They both fall on the same day.
The terms "Mardi Gras"(mär`dē grä) and "Mardi Gras season",<1><2><3><4><5><6> in English, refer to events of the Carnival celebrations, ending on the day before Ash Wednesday. From the French term "Mardi Gras" (literally "Fat Tuesday"), the term has come to mean the whole period of activity related to those events, beyond just the single day, often called Mardi Gras Day or Fat Tuesday.<1><2><3><4><5><6> The season can be designated by the year, as in "Mardi Gras 2008".<6>
The time varies from city to city, as some traditions consider Mardi Gras as the Carnival period between Epiphany or Twelfth Night and Ash Wednesday.<7> Others treat the final three-day period as being Mardi Gras.<8> In Mobile, Alabama, Mardi Gras events begin in November, followed by mystic society balls on Thanksgiving,<7><9> then New Year's Eve, formerly with parades on New Year's Day, followed by parades and balls in January & February, celebrating up to midnight before Ash Wednesday.<7>
http://en.wikipedia.org/wiki/Mardi_GrasShrove Tuesday is a term used in Ireland, the United Kingdom, Canada<1> and Australia<2> for the day preceding the first day of the Christian season of fasting and prayer called Lent.The word shrove is the past tense of the English verb shrive, which means to obtain absolution for one's sins by way of Confession and doing penance. Thus Shrove Tuesday gets its name from the shriving that English Christians were expected to do prior to receiving absolution immediately before Lent begins. Shrove Tuesday is the last day of "shrovetide", somewhat analogous to the Carnival tradition that developed separately in countries of Latin Europe. The term "Shrove Tuesday" is no longer widely known in the United States outside of Liturgical Traditions, such as the Lutheran, Episcopal, and Roman Catholic Churches. <3><4> Because of the increase in many immigrant populations and traditions since the 19th century "Mardi Gras" is much more widely-used.
The festival is widely associated with the eating of foods such as pancakes, and often known simply as Pancake Day, originally because these used up ingredients such as fat and eggs, the consumption of which was traditionally restricted during Lent.
http://en.wikipedia.org/wiki/Shrove_TuesdayNext:
Ash Wednesday