You may know why Holy Thursday is called Maundy Thursday, but do you know why the day before called Spy Wednesday?
Many Catholics, on hearing the name Spy Wednesday, assume that Spy must be a corruption or abbreviation of a Latin word. That’s a reasonable assumption: After all, the Maundy in Maundy Thursday (Holy Thursday) is an anglicization (by way of Old French) of the Latin mandatum (“mandate” or “command”), referring to Christ’s commandment to His disciples at the Last Supper in John 13:34 (“A new commandment I give unto you: That you love one another, as I have loved you”).
Likewise, the Ember in Ember Days has nothing to do with fire but comes from the Latin phrase Quatuor Tempora (“four times”), since the Ember Days are celebrated four times per year.
But in the case of Spy Wednesday, the word means exactly what we think it means. It’s a reference to Judas’s action in Matthew 26: 14-16:
“Then went one of the twelve, who was called Judas Iscariot, to the chief priests, and said to them: What will you give me, and I will deliver him unto you? But they appointed him thirty pieces of silver. And from thenceforth he sought opportunity to betray him.”
The beginning of Matthew 26 seems to place that event two days before Good Friday. Thus, a spy entered the midst of the disciples on Wednesday of Holy Week, when Judas resolved to betray our Lord for 30 pieces of silver.
Richert, Scott P. “Why Is Wednesday of Holy Week Called Spy Wednesday?” ThoughtCo, Jan. 28, 2019, thoughtco.com/spy-wednesday-3970805.