RE:volumeI may be mistaken, but the 3rd Friday of specific months is when "rebalancing of index contracts" takes place, as a rule of thumb, for American equities, both as individual stand-alone equities, but also if they are named components in specific stock indices' contracts, such as the XAU, and the HUI for just two examples.
That 3rd Friday timeline is when option settlement is determined and parties are either chased out because they are in a loss position called "out of the money", or assigned a marginable index contract, because they are "in the money" by the close on 3rd Friday. If one is in the money, said profitable option position holder has until Noon on Saturday to physically make up their mind if they are going to take the option premium and run to the bank with it, or bow out of the winning position before it gets converted to a contract which requires a hefty cash margin (sometimes 100%) to hold onto their winning hand.
This behavior affects not only individuals in various indices' holdings, but also mutual funds and hedge funds who hold specific stocks which comprise a fraction of the totality of the index contracts.
Sometimes companies are removed from their "index" portfolio mixture, and that removal giggles the total percentage of the whole of the remaining equities. According to the prospectus filings with the SEC (and I suspect Canadian Securities regulators as well, but don't know that for sure) .
When components are added or removed, the percentages of the others always is adjusted on these 3rd fridays, as mentioned earlier, but also, the MF, and the HF are required by their prospectus to buy or sell the security being added or removed. This creates a tremendous volume.
Another feature contributing to the volume fluctuation involves the required publication of the POSITION IMBALANCE "x number of minutes" prior to the close on 3rd Friday's. This is to give the massive money held by MF and HF prospectus players a chance to get the rebalancing/buys or sells squared away so that they are in compliance with SEC regulators and their governing prospectus of their investors.
The 19th was most assuredly the 3rd Friday in December, and that accounts for the volume spike you observed.
GL2Y