Why? Because good parties are intrinsic to our Catholic faith. The liturgical year is punctuated with a wide array of feast days and celebrations, many of which are Christianized versions of holidays that once closely tracked the agricultural calendar of planting and harvesting. The two largest and best-known feasts are, of course, Christmas and Easter, but there are also the two Christmas and Easter spin-offs, Epiphany and Pentecost. In addition, there's the feast of Mary, Mother of God (New Year's Day); Ascension Thursday; Corpus Christi; the feast of the Immaculate Conception; All Saints Day (with Halloween and the Day of the Dead); and, the most famous party of all, Mardi Gras, which has strayed far from its Catholic origin as the last celebration before the Lenten fast but still embodies a certain Catholic sensibility. Above all, every Sunday for Catholics is a feast day on which we celebrate Christ's resurrection. Only in Lent and the mini-Lent of Advent is it not party time, but even in these two seasons, there are exceptions for St. Valentine's Day, St. Patrick's Day, St. Nicholas's Day, and other feasts.
Of course, as the Church wisely realizes, feasts are more fun if preceded by fasts. The stricter the fast, the merrier the feast. Truly the Catholic tradition has mastered the art of well-timed, heavily scheduled, carefully orchestrated good times.