The S&P 500 closed above 2,000 for the first time ever, but traders say it will quickly be put to the test once the usually volatile month of September gets underway next week.

"I think the next spot that's going to be psychological is what happens when the perceived big boys come back from vacation.... A lot of people who are short this market say: 'Wait for Labor Day when people come in and sell,' " said Scott Redler, partner at T3Live.com.