Best time to buy plane tickets for December?

Hi, my sister and I are planning our first Japan trip in mid-December 2020 and planning to stay for 2 weeks. This is our first time traveling internationally and wanted to know when is the best time to buy plane tickets from Chicago to Tokyo? Should I buy it around the summertime? Thank you and I appreciate the help!

Get on some cheap flight mailing lists. Scott's Cheap Flights, Jack's Flight Club, etc. Book when they send you a good deal for the dates you are interested in. I don't know about flights from your airport but I've done this many times to Japan from two different UK airports and saved £hundreds every time. In my experience this will get you the cheapest possible flights without having to check Google Flights, Skyscanner, etc. every day. My next flight (June) cost roughly half of what it would have cost to book with ANA on the same day.

You can set up a flight alert on Google Flights that let you know when the price drops on your chosen trip.