use preposition the office is closed ___ second and forth saturdays

for ?

A better preposition would be "on."

The preposition that fits in the blank to complete the sentence is "on."

To find the correct preposition, we need to consider the construction of the sentence and the word "closed." In this context, "closed" is indicating when the office is not open.

Since the sentence is discussing specific days, the preposition "on" is used to indicate the days of the week on which the office is closed. Therefore, the correct sentence would be "The office is closed on second and fourth Saturdays."