E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
The day before Good Friday is so called from the Latin dies mandati (the day of Christs 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.