Right, well Mardi Gras and Fat Tuesday is basically the last celebration before 40 days or fasting, prayer and alms giving.
In history, today would be the day where people would eat all the fat in their homes, to prepare for the fasting up ahead.
Here's a brief account of why it is 40 days
Because forty days is a traditional number of discipline, devotion, and preparation in the Bible. Thus Moses stayed on the Mountain of God forty days (Exodus 24:18 and 34:28), the spies were in the land for forty days (Numbers 13:25), Elijah traveled forty days before he reached the cave where he had his vision (1 Kings 19:8), Nineveh was given forty days to repent (Jonah 3:4), and most importantly, prior to undertaking his ministry, Jesus spent forty days in wilderness praying and fasting (Matthew 4:2).
Since Lent if a period of prayer and fasting, it is fitting for Christians to imitate their Lord with a forty day period. Christ used a forty day period of prayer and fasting to prepare for his ministry, which culminated in his death and resurrection, and thus it is fitting for Christians to imitate him with a forty day period of prayer and fasting to prepare for the celebration of his ministry's climax, Good Friday (the day of the crucifixion) and Easter Sunday (the day of the resurrection).
