On Tuesday, Tesla stock gained 5%.
According to Business Insider the S&P 500 Index Committee decided to add Tesla stock to the index all at once on December 21.
Tesla's large market valuation is more than $500 billion.
The committee had considered either adding Tesla stock to the index in stages, or doing it all at once.
The committee's decision to add Tesla stock to its index in one fell swoop will create unprecedented demand for Tesla stock.