This is the 1785th most frequent Turkish word.
ayındaki
"Belonging to or in the month of."
Here, 'ayındaki' is used to specify something within the month of May, which is a time-related specification.
Yılın Mayıs ayındaki sıcaklıklar genelde yüksektir.
The temperatures in May of the year are generally high.
In this sentence, 'ayındaki' refers to a story happening within the month of September, mentioned in the book.
Kitapta geçen Eylül ayındaki hikaye oldukça dokunaklıydı.
The story in September mentioned in the book was quite touching.
This usage indicates something related to the month of December, specifically exam results.