Mark Hulbert writes June 1 at MarketWatch:

Monday’s trading will be the first opportunity stock investors in the U.S. will have to act on a major technical violation that occurred at Friday’s close: The breaking of the 200-day moving average.

This could result in an avalanche of sell signals hitting the market at Monday’s open, since many technical analysts use the 200-day moving average as the dividing line between bull and bear markets. They consider the primary trend to be up so long as the market is trading above its 200-day moving average, and that this trend turns to bearish whenever the market closes below this average—and that is what happened at Friday’s close.

Read more here.