For Catholics
Fat Tuesday is the day before Ash Wednesday when Lent begins.
The Legend of the Pancake was the housewife was making pancakes with the last of her lard when the church bells began to ring signaling the beginning of Mass and the start of the Lenten season.
Not wanting to be late for Mass and not wanting to waste the last of her Lard, as well as the indulgence of sweet treats, she flipped the pancake in her pan on the way to Mass.
This is at least the legend that inspired the Pancake race between Liberal, Ks and Onley, England.
For non-Catholics and Catholics alike, Fat Tuesday is the last day of Mardi Gras or (Fat Tuesday) in French. And now begins the 40 day fast. In modern times we only fast and abstain from meat on Ash Wednesday and the Fridays of Lent. Some older Catholics still observe by eating only one small meal a day, and abstaining from sweets, meat, etc during the 40 days that follow up until Easter.