Here are five things you must know for Tuesday, Dec. 26:

1. -- U.S. stock futures turned lower on Tuesday, Dec. 26, as traders returned  to their desks after the Christmas holiday.
 
Apple Inc. ( AAPL) fell 2.8% in premarket trading on a report that first-quarter demand for the iPhone X could come in under analysts' estimates.
 
Taiwan's Economic Daily reported the tech giant will slash its sales forecast for the iPhone X in the quarter to 30 million units from 50 million. The newspaper cited unidentified sources.
 
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Stock markets were closed Monday, Dec. 25, for Christmas. U.S. equities finished lower on Friday, Dec. 22, after Donald Trump signed the U.S. tax bill into law and lawmakers agreed to terms of a spending bill that would avert a near-government shutdown. Even with Friday's losses, stocks finished higher for the fifth week in a row.
 
The Dow Jones Industrial Average has gained more than 25% in 2017, and is on pace for its best performance since 2013.
 
Most European markets on Tuesday were closed, while Asian shares finished the session mixed.
 
The economic calendar in the U.S. on Tuesday is light, with only the Richmond Fed Manufacturing Index for December at 10 a.m. ET, and the Dallas Fed Manufacturing Survey for December at 10:30 a.m.
 
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2. -- Bitcoin was trading Tuesday at $15,269 on the bitstamp exchange, up 9.8%, after plummeting nearly 30% on Friday to below $12,000.
 
The digital currency has risen from less than $1,000 at the start of 2017 to as high as $20,000 a little more than a week ago.
 
Cryptocurrency entrepreneur Julian Hosp told CNBC that bitcoin's rapid rise isn't over yet, saying he thinks bitcoin could hit $60,000. But he also said he thinks bitcoin will fall to $5,000.

"The question is though, 'Which one is it going to hit first?'" Hosp told CNBC.

3. -- U.S. retail sales over the holiday period rose at their best pace since 2011, according to Mastercard SpendingPulse, which tracks both online and in-store spending.

Sales, excluding automobiles, rose 4.9% from Nov. 1 through Christmas Eve, compared with a 3.7% gain in the same period last year, according to the Mastercard unit, The Wall Street Journal reported. E-commerce continued to drive the gains, rising 18.1%.

4. -- Elon Musk's massive backup battery installed in South Australia already has paid dividends, according to reports.

When the coal-fired Loy Yang power plant in Victoria failed last week and led to a power cut, the battery kicked in and delivered as much as 100 megawatts of juice into the national electricity grid in just 140 milliseconds.

"That's a record and the national operators were shocked at how quickly and efficiently the battery was able to deliver this type of energy into the market," State Energy Minister Tom Koutsantonis told 5AA radio.

Musk plugged the world's biggest lithium-ion battery into an Australian state grid just a few weeks ago, delivering on a promise by the Tesla Inc. (TSLA)  CEO.

Musk said he would build the 100-megawatt battery within 100 days of contracts for the project being signed at the end of September or hand it over to the South Australia state government for free.
 
Tesla shares fell 0.6% in premarket trading.
 
5. -- United Airlines apologized and has given a $500 travel voucher to a passenger who accused the airline of giving her first-class seat to U.S. Rep. Sheila Jackson Lee, a Houston Democrat.

An airline spokeswoman told the Associated Press Monday that its internal systems show the passenger, Jean-Marie Simon, canceled her Dec. 18 seat from Houston to Washington, D.C. after a weather delay.

Simon denied that she canceled the flight. She was given a seat in Economy Plus on the flight, and told the Houston Chronicle she saw Jackson Lee sitting in the seat that was assigned to her.

United said it upgraded Jackson Lee automatically and not because she was a member of Congress. The congresswoman saids in a statement that she didn't ask for anything "exceptional or out of the ordinary."

Shares of United Continental Holdings Inc. (UAL) declined 0.4%.

This has been updated from 5:42 a.m. ET.

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