• Stock futures were lower in premarket trading Friday.
• Nike said sales during its holiday quarter fell 9% and warned of another hit ahead.
• London's Heathrow Airport is closed Friday after a massive nearby fire caused a power outage.

Here are five key things investors need to know to start the trading day:

1. Slip 'N Slide

Stock futures were lower in premarket trading Friday as the S&P 500 and Dow Jones Industrial Average look to post a positive week with one trading day left. Futures tied to the S&P 500 and futures tied to the Dow Jones Industrial Average were down about 0.3% each while Nasdaq 100 futures slipped 0.4%. As of Thursday's close, the S&P is up 0.42% on the week. The broad market index would snap a four-week losing streak if it can clinch a gain for the 5-day period. Follow live market updates.

2. Uncertain footing

Nike said sales during its holiday quarter fell 9%, dragged lower by a 17% sales decline in China, and it warned of another hit ahead. Executives said during a conference call Thursday that sales for the current quarter are expected to be down by a double-digit percentage as the company grapples with tariffs and sliding consumer confidence. Nike's been attempting a turnaround under recently installed CEO Elliott Hill, and expectations were low for the quarterly report. Nike shares were about 5% lower in extended trading.

3. Heathrow closed

London's Heathrow Airport is closed Friday after a massive nearby fire caused a power outage. Heathrow is Europe's busiest airport, and the closure could affect some 145,000 passengers, according to aviation-data firm Cirium. Flight-tracking site Flightradar24 reported 679 flights were scheduled to land and 678 flights were scheduled to take off from Heathrow on Friday. The hub will remain closed until at least 11:59 p.m. local time. "We expect significant disruption over the coming days," a spokesperson for the airport said in a statement.

4. Out of many, one

More U.S. households are investing in the stock market than ever before, making the appetites of individual investors a crucial market driver. According to JPMorgan, 60% of U.S. equities are now held by individuals rather than institutions. There are early signs that retail investors are getting spooked by volatility and economic uncertainty, with U.S. households estimated to hold 42% of total financial assets in equities during the first quarter, slightly lower than record levels seen during the previous period.

5. Sell-tics

The Boston Celtics are coming under new ownership — at an impressive price tag. The NBA team, currently owned by Boston Basketball Partners, which is led by the Grousbeck family, is selling to a group of investors led by private equity executive Bill Chisholm at a valuation of $6.1 billion. It's the highest sale price for a U.S. sports team in history. If the deal is approved, the sale will be completed this summer.

– CNBC's Hakyung Kim, Gabrielle Fonrouge, Leslie Josephs, Monica Pitrelli, Yun Li, Russell Leung and Jacob Pramuk contributed to this report.