Mardi Gras, which is French for “Fat Tuesday,” is the last day in a season called Carnival and the day before Ash Wednesday, which marks the beginning of Lent. Depending on the location, the Carnival season is two weeks in length and is characterized by merrymaking, feasting, dancing, masquerades, and general licentiousness. The carnival season is typically celebrated by Catholic countries of Southern Europe and Latin America.
How Mardi Gras relates to Lent is as follows. Lent is a season of fasting, penance and preparation for Easter. Christians who observe Lent usually do so by abstaining from certain foods or activities they enjoy with the express purpose of focusing that time to prayer, penance, and almsgiving. There is also regular fasting prescribed during Lent. Since Carnival leads up to Lent, it can be rightly seen as the indulgence before the fast. Think of it as one last “binge” before giving something up for 40 days.
What does the Bible say about all of this? There is nothing in the Bible that either explicitly or implicitly suggests that early Christians observed either Lent or Carnival. Let’s first look at Carnival a little more closely. We would be very hard pressed to find biblical support for any kind of fleshly indulgence such as is practiced during Carnival, especially on Fat Tuesday. The Bible expressly forbids drunkenness, sexual fornication, and debauchery of any kind. The best verse for this can be found in Romans 13:13–14, “Let us walk properly, as in the day, not in revelry and drunkenness, not in lewdness and lust, not in strife and envy. But put on the Lord Jesus Christ, and make no provision for the flesh, to fulfill its lusts.” We are exhorted to be sober-minded and alert, not engaging in debauchery. The idea of a binge of sinning before a time of consecration to the Lord is completely ridiculous and utterly unscriptural.
 Got Questions Ministries. (2010). Got Questions? Bible Questions Answered. Bellingham, WA: Logos Bible Software.