On what situations we should say "no" to your managers?

I dont think you will need to say "No" to a professional manager, as he will definitely know how to avoid such answers from his subordinates. 

I agree with Mr. Venkitaraman, ethics matter, so do not hesitate to say a "NO" to your manager whenever he asks you to do something unethical ...

When you have other deadlines. Saying "No" is not an easy thing. You can say it in ways that will make it clear that you have the best intentions in mind and will avoid negative consequences.

100% agree with Mr Divyesh. 

I sometimes say "No" when they keep asking me to come to work on the weekends

To me, If the instructions are not ethical for a corporate culture-there is no mistake in saying a big NO.

 Instead of saying No outright, it might be better to ask the manager to schedule your time for the open tasks in hand. After reviewing he may postpone some tasks to later time to accommodate something important and urgent.. 

I agree with experts specially if you have been asked to fire or suspend somebody unjustly.

Other than unethical orders you should show some flexibility ...

Every professional should refuse to contribute towards any unethical, immoral or illegal activities.

"No" in better than nothing /......bcz nothing is trying to do something officially. then it will never say " No" . always trying only.