The 8:20am CME Open is such a relevant event for the bond market sometimes that it has its own entry in our knowledge base.  On any given morning, it's possible to see traders lined up (figuratively) to buy or sell as soon as 8:20am hits. 

This is all the more likely on the first and final days of any given week or month. Here on August 1st, a small but noticeable majority of traders were lined up to sell.  The spike in volume and selling pressure at 8:20am were obvious and they added to what had been modest weakness overnight.  

The rest of the session is likely to take its cues from the 10am econ data.