Is a postdoctoral scholar expected to work during the summer? I am considering applying for postdoctoral positions in the US.
However, I would like to find more information about
how it is like to be a postdoctoral scholar.

Question 1:
Is a postdoctoral position a full-year position (12 months)
or an academic-year position (9 months)?

Question 2:
How should I interpret postdoctoral salaries?
For example, an MIT postdoctoral information page (see link)
lists the minimum postdoctoral salary
for a person with 0 years of experience at $42,000.
Is this salary for 12 months, or for 9 months?
In general, postdoctoral positions—unlike faculty positions (in the US)—are considered to be 12-month positions, with the salaries determined accordingly. You are not normally expected to provide your own funding for the summer months, nor are you expected to find outside employment.

If this is not the case, then it should be explicitly mentioned in the advertisement for the position. However, this is so rarely encountered that it would be very much outside the norm for postdocs.