When should I sell Futures contract on DJ Industrial Average ETF Trust?

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 Futures contract on DJ Industrial Average ETF Trust before Nov 30, 1899.

What is Open interest? How can I use it to trade Futures contract on DJ Industrial Average ETF Trust?

Open interest is the number of contracts held by traders in active positions — they're not closed or expired. For Futures contract on DJ Industrial Average ETF Trust this number is 0. You can use it to track a prevailing market trend and adjust your own strategy: declining open interest for Futures contract on DJ Industrial Average ETF Trust shows that traders are closing their positions, which means a weakening trend.