1) English-language idiom 2) Midnight 3) Midnight, to some 4) Stock market 
With a modern literal meaning of `midnight,` the term witching hour refers to the time of day and night (12:00 a.m. or 12:00 p.m. is commonly speculated) when creatures such as witches, demons, and ghosts are thought to appear and to be at their most powerful and black magic to be most effective. It may be used to refer to any arbitrary time of .....[Investing] In the context of investing, the term `witching hour` refers to the last hour of stock trading between 3pm (when the bond market closes) and 4pm EST (when the stock market closes) and can be characterized by higher-than-average volatility. ...
Traditionally, this is midnight.