Etymology
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Good Friday (n.)
the Friday before Easter, c. 1300, from good (adj.) in Middle English sense of "holy, sacred," especially of holy days or seasons observed by the church; the word also was applied to Christmas and Shrove Tuesday. Good Twelfthe Dai (c. 1500) was Epiphany (the twelfth day after Christmas).
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Maundy Thursday 

Thursday before Easter, mid-15c., from Middle English maunde "the Last Supper" (c. 1300), also "ceremony of washing the feet of poor persons or inferiors, performed as a religious rite on Maundy Thursday" (early 14c.), from Old French mandé, from Latin mandatum "commandment" (see mandate (n.)); said to be so called in reference to the opening words of the Latin church service for this day, Mandatum novum do vobis "A new commandment I give unto you" (John xiii:34), words supposedly spoken by Jesus to the Apostles after washing their feet at the Last Supper.

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