The day before Good Friday is so called from the Latin dies mandati (the day of Christ's great mandate). After He had washed His disciples' feet, He said, “A new commandment give I unto you, that ye love one another” (St. John xiii. 34).
Spelman derives it from maund (a basket), because on the day before the great fast all religious houses and good Catholics brought out their broken food in maunds to distribute to the poor. This custom in many places gave birth to a fair, as the Tombland fair of Norwich, held on the plain before the Cathedral Close.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894
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