How do I find all the drastic changes in the stock and fetch news for those days of the stock?
I would say that it probably has a great deal to do with what you are planning on doing with the results of your model. For example, if you are looking for news that corresponds to a move of the stock price by more than 3% on that day, you would only want to look at those days. Alternatively, if you are looking for 'abnormally large' movements in the stock price, you might consider taking the mean and standard deviation of your day to day movements and only considering those days that are more than two or three standard deviations from the mean.

In terms of how to get the news for a particular day, here is a python tutorial that may be useful:
https://www.w3resource.com/python-exercises/basic/python-basic-1-exercise-8.php