The month of September has the dubious honor of being the worst performing month for the S&P 500. Blame it on post-summer depression or portfolio manager rebalancing; either way markets tend to trend lower into quarter end. The bright side is there's a silver lining to September's swoon. If you're willing to take the long view, you're typically rewarded by the end of November.

For those with cash, September's historical downturn has served as a great buying opportunity.
The S&P 500 has finished November higher than it started September in 9 of the past 10 years, generating average and median returns of 1.95% and 4.07% respectively. That suggests you shouldn't get too nervous if the market falls this month.

Source: Seasonal Investor Database

Instead, you may find it more helpful to consider buying companies in the consumer discretionary sector. Retailers are some of the best performers heading into fall as people digest back-to-school season and anticipate holiday spending. Over the past decade, the Consumer Discretionary SPDR ETF (XLY) has traded higher in 9 of the past 10 3-month periods ending November, increasing an average and median 2.88% and 4.78%, respectively.

These two retailers should be on your radar
There are two large-cap consumer discretionary companies you should watch this month. Costco (COST -1.23%) and Urban Outfitters (URBN -0.38%) have both posted strong returns through November, suggesting September may be a good time to buy.

Walk into Costco and the first thing you'll notice is the sheer size of everything
Big buildings, big displays and big shopping carts -- one steered by each hand -- are common at Costco. The company pioneered bulk buying and shoppers have been picking pallets since 1976. The company --one of the few that still reports monthly numbers -- reported that sales at its 632 warehouse style stores improved 7% to $7.9 billion in July from last year. That brought fiscal year-to-date sales to nearly $95 billion, up 8% from 2012.

Part of this sales growth is tied to Costco's growing international store count. Of the five warehouses opened in fiscal Q3, four were overseas. Of the nine stores slated to open in Costco's fiscal fourth quarter, six are in overseas markets. Those additions will bring the total of new stores opened in fiscal 2013 to 28, up from 16 opened in 2012.

Those new stores set Costco up for success heading into its new fiscal year.  Over the past decade, Costco's shares have traded up in 9 of the past 10 3-month periods that started in September and ended in November, producing average and median returns of 9.64% and 11.55%, respectively. 

There's no more fickle shopper than a teenager
In any given season, fast changing fashion trends significantly punish or reward apparel retailers. That's no surprise to Urban Outfitters. The company has been navigating teen fashion trends since 1970.

Urban Outfitters entered this back-to-school sales season with tailwinds. Sales at stores open more than one year improved 9% last quarter, suggesting the company has traction with young buyers. Those buyers helped quarterly sales climb 12.2% to $758 million, generating $0.51 in earnings per share, 21.4% more than a year ago.

One of the company's fastest growing brands is Free People, which tends to target a bit older crowd, mid to late 20's. The company continues to open new Free People stores after seeing the brand's sales climb more than 30% to $97 million last quarter. With only 83 Free People stores, there's plenty of room to grow, given that there are 222 Urban Outfitters stores and 182 Anthropologie brand stores.

This debt-free company is perennially shareholder-friendly too. Urban Outfitters repurchased 20.5 million shares of stock in the fiscal year ending 2012. And, the company recently approved a 10 million share buyback.

Given that the retailer has $298 million in cash and short-term investments on its balance sheet, the buyback trend is likely to continue to support share prices. Over the past decade, Urban Outfitters' shares have moved higher in 9 of the past 10 years through November, producing an average and median return of 15.7% and 12.85%, respectively.

Conclusion
The market often makes powerful moves that can test the patience of any investor. If September proves to be challenging, keeping your hand steady and using weakness as an opportunity to buy shares in quality companies like Costco and Urban Outfitters may prove profitable.