So what's an investor to do? Like most well-known market truisms, successful investing strategies are not as straightforward as pithy sayings.

In our view, the best "strategy" is to focus on the market's actual price action and internals; any signs of a reversal lower will show up in the price action before anywhere else. So how's the market look now?

With the S&P 500 trading above both the (rising) 50- and 200-day moving averages, the NYSE "Common Stock Only" Advance-Decline line hitting a new high, and the MACD trending higher above both the "0" level and its signal line, the path of least resistance remains to the topside for U.S. stocks in our view:

Source: Stockcharts.com

...and it certainly doesn't hurt that the bullish view is also a contrarian trade to my father-in-law's concerns about the market's seasonality!

Obviously, if the evidence changes, we'll change our view, but for now, we see no reason to doubt the established bullish trend in stocks.

*It's worth noting that the ETF, which is based on the work of research of CFRA CIO Sam Stovall, actually just rotates into different sectors through the summer months, rather than sell stocks completely.