9:30–9:40 a.m. Stocks that open higher or lower than they closed typically continue rising or falling for the first five to 10 minutes…
9:40–10:00 a.m. …before reversing course for the next 20 minutes—unless the overnight news was especially significant.
10:00 a.m. In either case, you should know by this time whether the opening trend will hold or reverse itself.
3:00–4:00 p.m. While price trends can break either way in the opening hour, they tend to build consensus in the closing hour—barring big news during the trading day.