The nearest expiration date for E-mini S&P 500 Futures (Jun 2015) is Jun 19, 2015.
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 E-mini S&P 500 Futures (Jun 2015) before Jun 19, 2015.