Fill in the blank with the appropriate preposition:

When I was a small boy ____ Bali, the school picnic was one of the major events ____ the calendar.

in/on

The correct answer is OPTION 4 --  in and on.

The preposition in is used to indicate location or position, and it can also be used to indicate involvement or participation in something. In this case, when I was a small boy in Bali, the school picnic was one of the major events on the calendar, which means that the school picnic was one of the most important events that happened during the year.

The other options are incorrect.

• At is used to indicate something that is being discussed or talked about, or it can be used to indicate movement or placement near something.
• On can also be used to indicate location or position, but it is not the most appropriate preposition to use in this case.