Shall I keep windows and blinds open during hot summer days?
Freeman's comment is exactly right.  The optimum approach depends on delta temperature, humidity, breezes, how long the sun shines directly into a window, etc.
Get a couple thermometers (and humidity monitors if you can) and record the variation from dawn to dusk for several days with windows closed, several with windows open, etc.
If your area cools down nicely at night, then be sure to keep windows open all night, then close them up in the AM to preserve the cool air in the room, for example.