Is it possible that I am not allowed at all in Czech Republic because I have 8 stay days on visa and the flight back is one day later?
Yes, it's possible. Your visa is supposed to be valid for the whole duration of the stay and your return flight is prima facie evidence that you intend to stay longer than allowed (and also longer than what you claimed on your original application, which is not good either).
On the other hand, it's also possible that the border guards do not even notice or decide that one day is not serious enough to deny entry.
Even if you gain entry, it's also (theoretically) possible to be fined on exit (with the same caveats) and your passport will in any case bear stamps showing you did not fully respect the conditions of your visa, which might be a problem when you apply for another visa in the future (even a non-Schengen visa).
Obviously, overstaying by one day/a few hours is a relatively benign infringement and less likely to lead to serious consequences than staying for weeks or immigrating illegally but it's still a bad idea.