Does the flavour of coffee beans improve after opening the bag? After I open a bag of beans, the flavour of the coffee seems better after the bag has been opened a day or more.  

Does the flavour of coffee beans improve for a period of time after the bag has been opened?  Perhaps I'm imagining it!
That is almost the opposite of my experience, after opening a bag of beans, they have noticeable less flavour after a couple of days. Coffee beans are more resistant than ground coffee though, as that decays in a couple of hours. Obviously, if beans are exposed to air for long enough, their quality is decreasing.

However there is an effect that improve them at first, simply because some of the processes from the roasting takes time to finish, and may require several days in order to do so. They may even require exposure to air (oxygen) in order to oxidise some of the compounds causing the flavour. Keep in mind that even though some of the compounds improve, other does not. The most notable degradation in the first days is caused by the most volatile aroma compounds evaporating.

This graph shows the stalling of coffee as a function of time. Note that this is just a visualisation of the aroma compounds evaporating, not necessary a measure of quality. The remaining compounds are also changing, and some of them improves over time.