In the ASK Graded Readers, which present beginner-appropriate short stories, there is a frequently occurring pattern of writing with ~ます during the 'setup' of the story, and then switching to 〜ました when something important happens. The sentence containing the tense switch often starts with ある日 or similar. The story then resumes using the 〜ます form once we leave this major event.
What is happening with the tenses here--why is the story told partially in non-past and partially in past? Is it typical for Japanese stories to be told in this way, and is this pattern also used in spoken Japanese?