From the folks at Bespoke Investments, a market analysis firm to which I am a subscriber (www.bespokeinvestments.com)…

As shown, below, had you bought SPY at the close every trading day this year and sold it at the next open, you’d still be up 9.14% this year (and another 1% today!).  Had you only bought SPY at the open every trading day and sold it at the close, you’d be down 7.77%!  Over the last month or so as the market has collapsed, the intraday selling has been extreme, to say the least.  Earlier this week we noted that 70% of the time that the market has been open over the last three weeks, it has been down, which is not seen very often.  The key test for the market today will be whether it can finally hang on to early strength for a change.