When should I buy air ticket to japan?

Despite a few posts on here prices do go both up and down. When I went to japan a few weeks ago, I bought my tickets about 5 months in advance. Between the purchase date and my flight date the price of the airfare did drop a little (ie approx $50-100), before going back up again. With that said most of the time prices will go up, especially when it gets closer to flight time. Although there may be a sale or something that will have lower prices, it is much more likely that the price will keep going up. Also the longer you wait the less chance you will have of getting 2 seats next to each other if you are traveling with someone. I personally recommend booking sooner than later and not risk of waiting for a better price.