There is an overall upward bias for the stock indices, as companies that perform poorly are dropped from an index while companies that are doing well are added. The S&P 500 has annualized yearly returns of 10.6% over the last 100 years. Yet the index is always fluctuating, and seasonality is the study of how an asset performs in various parts of the year.  The S&P 500  has historically not performed well in June. Over the last 20 years, June has been a weaker month. The average return for the S&P 500 in June is -0.1%. It is also essentially a coin flip as to whether the index will rise or fall in June. 12 of the last 20 Junes have been positive, while eight have been negative. The months that were down have outstripped the gains in the positive months which is why the average is negative. 

Over the last 20 years, June and September are the only months of the year with negative average returns for the S&P 500. September is worse than June.

As for other indices:

The NYSE Composite has only moved up in nine out of the last 20 Junes, and has averaged a loss of -0.3%.

The Nasdaq 100  has faired only marginally better, moving up in 10 of the last 20 Junes for an average gain of 0.1%.

Compared to other months, June is historically one of the weakest for these indices as well.

What does that mean for investors and traders?

All traders and investors should stick to their trading and investment plans. Seasonality is just one data point to possibly include in market analysis. Seasonality is also just an average of what has happened historically. Sometimes the price moves up in June, other times it moves down. Yet on average it hasn't been great. 

If it turns out that the indices pull back in June, that provides an opportunity to scan for strong companies that may also see a pullback and thus present a buying opportunity. July has been one of the best months of the year, historically, so June pullbacks may provide entry points for July rallies.

Given this data point, there is also the option to more aggressively protect profits if stock holdings or the indices start to turn lower. If trades are working, stick with them, because seasonality is not a crystal ball that reveals what will happen this year.