From Halifax, NS the best time to book for January is in between October to January. As low as $300 return taxes in when I checked this fall for January 2017. We are going Jan 27-Feb 7, 2018.
I use the cell phone app called Hopper! It keeps track of the flight route you are interested and keeps an eye on prices and the best time to buy. We just saved about $150.00 for 2 for travels this summer. Highly recommended.
I'd start tracking fares now and jump when you find a deal. I'm going end of November. Haven't booked yet. I did just book the other day for late Aug trip.