The story
The idea of witching for these days is from the unusual patterns of trading that can occur ahead of the contract expiries.
Etymology from Wiktionary, CC BY-SA 4.0.
triple witching
Today it means: Simultaneous expiry on US markets of stock index futures, stock index options, and stock options, which took place on the third Friday of March, June, September, and December. (With the introduction of single stock futures those days are now quadruple witching.) Full definition →