NEW YORK ( TheStreet) -- Jim Cramer answered viewers' Twitter (TWTR) questions from the floor of the New York Stock Exchange, commenting on Qorvo (QRVO - Get Report), Celgene (CELG - Get Report) and Boston Scientific (BSX - Get Report), and on why September is a tough month on Wall Street.

A viewer asked, what are the factors that have caused September to be such a bad month historically on Wall Street? Cramer responded that there are several reasons, including the fact that October is the end of the fiscal year for a lot of mutual funds, and investors like to get out of the market ahead of that.

Cramer also said some people take profits after a summer rally, and September is when many companies issue earnings pre-announcements, particularly technology companies.

Another viewer asked about Qorvo. Cramer said the company had a disappointing quarter, which brought down other stocks, including NXP Semiconductors (NXPI - Get Report), which he likes better, and Skyworks Solutions (SWKS - Get Report), which he said to buy in the $70s.

One viewer asked about a good entry point for Celgene, which Cramer said is a buy at $113 or below.

Cramer said that Boston Scientific is a good company, and that he likes the whole group, including C R Baird (BCR), Edward Life Sciences (EW - Get Report). Medtronic (MDT - Get Report) and St. Jude Medical (STJ).

Finally, Cramer was asked at what point in the market would he commit all his cash reserves and buy stocks because the prices are so good. Cramer noted that when the S&P 500 trades at 14 times earnings, that would be a great opportunity.

"I think you have to be ready, that could happen," he said, adding the index is now trading at 16 times earnings.

Cramer spoke with TheStreet's Rhonda Schaffler on the floor of the New York Stock Exchange. Send your Twitter questions to Cramer using #Cramerq.

Action Alerts PLUS, which Cramer co-manages as a charitable trust, owns shares of Twitter.