This is the 5882nd most frequent Turkish word.
tatilini
"Her holiday" or "his holiday" (possessive form of "tatil").
Here, 'tatilini' is a possessive form referring to Ahmet's vacation.
Ahmet tatilini İspanya'da geçirdi.
Ahmet spent his vacation in Spain.
In this sentence, 'tatilini' refers to the subject’s vacation, indicating possession.
Tatilini ne zaman planlayacağını söylemedi.
He did not say when he would plan his vacation.
'Tatilini' here is asking about the possession of the vacation concerning the listener.