When should I sell Dollar Based Currency Pairs Japanese Yen/US Dollar?

The best time to sell a futures contract is when you've already made money on the investment, but still have plenty of time left before the expiration date. Thus, a good option would be to sell Dollar Based Currency Pairs Japanese Yen/US Dollar before Nov 30, 1899.

What is Open interest? How can I use it to trade Dollar Based Currency Pairs Japanese Yen/US Dollar?

Open interest is the number of contracts held by traders in active positions — they're not closed or expired. For Dollar Based Currency Pairs Japanese Yen/US Dollar this number is 0. You can use it to track a prevailing market trend and adjust your own strategy: declining open interest for Dollar Based Currency Pairs Japanese Yen/US Dollar shows that traders are closing their positions, which means a weakening trend.