It is mostly dependent on base rate, demand and supply and number of days left.
December 21st being holiday season, lots of folks who wanted to buy tickets cheaper would have tried early. Demand may have slumped then as most people who had planned, would have bought. (This may not always happen). So, price may be cheaper now. And it may again go up if demand increases as travel date gets nearer..
For non holiday season, I generally buy 30 to 20 days before travel. For holiday season, I have never figured out good time. Usually earliest is better, but sometimes it doesn't work that way.