That is the day Russell Investments sets the final update for the annual reconstitution of its indexes after the close of trading. This rebalancing creates a surge of liquidity as investors readjust portfolios and try to take advantage of dislocations in stock prices.

"If you are looking to significantly alter your basket or portfolio, or liquidate or try to buy on the dip... this is a good place to go to do that because there is going to be more volume in the last few minutes of trading on the day of the Russell then there might be over the course of a whole day on another day," said Gordon Charlop, managing director at Rosenblatt Securities in New York

This year's changes do not contain an addition as dramatic as the inclusion of Berkshire Hathaway was, but there are a number of other, smaller alterations.

According to Nasdaq, approximately 687.9 million shares representing $9.5 billion in value were traded in the closing 1.15 seconds across the nearly 2,200 Nasdaq-listed stocks in 2012. Credit Suisse estimates $42 billion in gross trading across the Russell 1000 and Russell 2000 indexes this year.

Because of the massive trade the reconstitution generates, Russell announces changes in its methodology for inclusion in the indexes months in advance, as well as a staggered release of new additions and deletions. The first of those changes was released on Friday.

"To the extent we know the actual trading activity can occur in a fairly short period of time, that is why we try to give as much information evenly to the marketplace within a specified time frame," said Rolf Agather, managing director of research and innovation at Russell Investments in Seattle, Washington.

Both the Nasdaq and New York Stock Exchange, a unit of NYSE Euronext, have contingency plans in place in case of unusual market conditions resulting from the rebalance. They include a possible system interruption, which could force exchanges to re-route trades to backup systems.

Data used by Russell to determine eligible securities, including market capitalization, is based on the last trading day in May. A preliminary list of additions and deletions was released on June 14, and updated lists will follow on June 21 and 28 to allow investors to try and predict the final changes.

In past years, changes in Russell's methodology resulted in Berkshire Hathaway Inc. and Tyco International Ltd being eligible for inclusion. According to the preliminary list of additions, Tyco is being removed from the Russell 3000 index, because it is now based in Switzerland, not the United States.

Notable preliminary additions to the Russell 3000 include Burger King Worldwide Inc., Zale Corp. and Pacific Sunwear. Preliminary deletions include Dolan Co. and Discovery Laboratories Inc.

This year, the methodology changes are considered relatively minor. That leads many analysts to believe the reconstitution may result in roughly the same amount in gross trading -- in the $40 billion range -- as in 2012.

"It's not uncommon for Russell to change some of its rules from year to year, and that usually in turn trickles down to certain stocks leaving or entering the index, which causes a little more turnover than usual - but we don't have that this year," said Stephen Casciano, a strategist on Credit Suisse's Trading Strategy team in New York.

One potential hiccup for investors is that Russell adjusted the date for the final rebalance to the last day of the quarter, preventing fund managers who engage in "window dressing," the practice of selling underperforming stocks and buying outperformers, to enhance the appearance of their portfolios.

Russell normally holds the final rebalance a week before the last day of the quarter. But it was forced to bump it back a week due to the quarterly settlement and expiration of four different types of equity futures and options contracts on June 21 - known as "quadruple witching" - which can create volatility.

Even though the rebalance is expected to be more subdued than in years past, investors tend to be on guard, given the sheer volume at the end of the session.

"If you wanted to be the contrarian you would say, 'Wow, there is not a lot of interest,' or maybe you would see some volatility because not too many people are paying attention to this," said Steve Desanctis, small-cap strategist at Bank of America Merrill Lynch in New York.

"If you keep saying 'it's not a big deal,' then it's not a big deal - until it's a big deal."

(Reporting by Chuck Mikolajczak; Editing by Dan Grebler)

By Chuck Mikolajczak