The fasting season that traditionally begins after Carnival is Lent.
Carnival developed as a period of feasting and celebration before Lent, the Christian season of penitence leading to Easter. In the Western calendar, Carnival reaches its climax on Mardi Gras, or Shrove Tuesday. Lent begins the next day, Ash Wednesday, so Carnival and Lent form contrasting parts of the liturgical year.
The traditional connection was practical as well as religious. Before Lent, communities used up rich foods such as meat, butter, eggs and other animal products that might be restricted during the fast. This helps explain food-centered customs including pancakes, pastries and elaborate public feasts.
Lent is commonly associated with 40 days, recalling Jesus’ fast in the wilderness. The exact counting differs among Christian traditions because Sundays may or may not be included. Carnival customs also vary widely and are not always religious in modern practice, but their traditional calendar placement before Lent remains widely recognized.
Advent is another Christian season, but it precedes Christmas rather than following Carnival.