Question: No longer officially marked by the Catholic Church, Quinquagesima Sunday still shows up in some liturgical calendars. What is Quinquagesima Sunday?
Answer: Quinquagesima Sunday is the final Sunday before the start of Lent (the Sunday before Ash Wednesday), which makes it the seventh Sunday before Easter. Traditionally, it was the third of three Sundays (following Septuagesima and Sexagesima) of preparation for Lent.
Quinquagesima literally means "fiftieth." It is 49 days before Easter, or 50 if you count Easter itself. (Similarly, Pentecost Sunday is said to be 50 days after Easter, but the number is calculated by including Easter in the count.)
When the Roman Catholic liturgical calendar was revised in 1969, the three pre-Lenten Sundays were removed; they are now denominated simply as Sundays in Ordinary Time. Septuagesima, Sexagesima, and Quinquagesima are all still observed in the celebration of the traditional Latin Mass.
For more information about Quinquagesima and the other pre-Lenten Sundays, see "What Is Septuagesima Sunday?"