That is a tough call. Normally, I would say wait until November or so before booking tickets for January, but January 2 is right at the end of the peak Christmas travel season, so seats will be scarce and prices expensive. And since it is a Saturday, WestJet is operating a vastly reduced schedule, which will only drive up the demand for seats more. WestJet is almost sold out of $199 seats as it is. There are usually some seat sales offered for the Christmas travel season, but they don't appear that often and usually only after the airlines know which flights will have empty seats. Obviously, they would prefer to sell out their flights at regular fares, if possible.
Do you have to fly on WestJet? Air Canada's flights are wide open on January 2 -- all of their flights are available at $199, and they have more scheduled that day. If you aren't too concerned about which airline you fly or exactly what time you fly, I'd wait a while and hope for some sort of a seat sale. If you must fly on WestJet or at a particular time, you might want to book your seats now. Any seat sale offered on January 2nd isn't going to be that great of a deal, anyway -- maybe $20 off per ticket.