This regarding the Russell 3000, from 2015 but appropriate.
It comes but once a year, and Friday is like a holiday for many Wall Street stock traders: it's Russell rebalance day.
Russell Rebalance Day — Does It Influence Small Stock Moves?
The annual reshuffling of the Russell 2000 Index of small-capitalization stocks tends to be the heaviest daily trading volume of the year. That's because FTSE Russell , a unit of London Stock Exchange (LSEG), implements the roster changes for which stocks belong in large-cap Russell 1000, Russell 2000, and the combined Russell 3000 indexes. Volumes crescendo at the end of the day, typically the last five minutes, because funds that follow these indexes aim to match their portfolios with end-of-day prices for stocks that are entering or exiting the indexes.
Anticipation for benchmark additions tends to help prices of stocks headed on to the new indexes. Colas finds that, this year, the 120 or so companies that are being added to the Russell 2000 have gained, on average, 11% since the start of May. Meanwhile, shares of the average company that leaving has declined 2%. https://www.barrons.com/articles/russell-rebalance-day-does-it-influence-small-stock-moves-1435336799