Personally, I think it's just too hard to predict whether a fare will fluctuate by $100. Even if someone were to offer a prediction, what is the percentage chance of it being correct?
I can tell you that ...
A) Calgary-Toronto is one of the top 5 routes traveled from YYC, and with 2 airlines monopolizing this popular route full-time, prices don't generally budge a whole lot, and genuine sales are rare, even on low demand dates.
and
B) The dates between December 20 and January 7 are where price drops go to die. Of the entire year, this is the date range the airlines are the most reluctant to relent on their prices.
and
C) Promo codes from WestJet/AC will never apply to Christmas dates. Nor will WestJet's Blue Tag Thursdays.
There's probably a much better chance of a price going up by $100 around Christmas dates, than down by $100. If I were betting on it at a casino, I'd probably put my money on Up.
But that being said, one just never knows! Even the airlines don't know, until they see what the demand has been like at the price levels they've set for individual dates.
You could look at booking with Flight Network. I've heard genuinely positive reports from people about their 'Price Drop Protection' (which are often a marketing scam that are impossible to redeem). Might want to make sure your fare would be covered by their terms & conditions though.