Today’s reading Zechariah 8:1-23 calls for a “fast of joy and gladness”. Like Lent is the fast before Easter, Advent used to be the fast before Christmas. Any major Christ-day has a preparatory fasting period before it. Maybe you can get a sense for what a “fast of joy” is when you consider the history of Stollen according to wikipedia:
The Advent season was a time of fasting, and bakers were not allowed to use butter, only oil, and the cake was tasteless and hard.
In the 15th century, in medieval Saxony (in central Germany, north of Bavaria and south of Brandenburg), the Prince Elector Ernst (1441 – 1486) and his brother Duke Albrecht (1443–1500) decided to remedy this by writing to the Pope in Rome. The Saxon bakers needed to use butter, as oil in Saxony was expensive, hard to come by, and had to be made from turnips, although we now know this was a healthy option.
Pope Nicholas V (1397–1455), in 1450 denied the first appeal. Five popes died until finally, Pope Innocent VIII, (1432–1492) in 1490 sent a letter to the Prince, known as the “Butter-Letter” which granted the use of butter (without having to pay a fine) – but only for the Prince-Elector and his family and household.
Others were also permitted to use butter, but with the condition of having to pay annually 1/20th of a gold Gulden to support the building of the Freiburg Minster. The ban on butter was removed when Saxony became Protestant.