Spy Wednesday is the day before Holy Thursday, and it's all about ... Judas, the apostle who betrayed Jesus for 30 pieces of silver. "Spy" in this case means to ambush or scare and refers to the way Jesus was captured by Roman soldiers who had been tipped off about his location.
I learn from Wikipedia that Spy or Holy Wednesday services are still held, often with a Tenebrae service, in which candles are extinguished until only one remains.
In Williams' novel, Spy Wednesday is the day when the rain finally stops in the fictional village of Faha. Much of the action revolves around this day.
The rain has stopped here, too, and the sun is shining on the downed branches and trees from last night's wicked storm. It's placid here for the moment. A time to learn about an old rite — and meditate on an old wrong.
(A Spy Wednesday process in Spain. Courtesy Wikipedia)