This is the 11168th most frequent Turkish word.
döneceği
"Döneceği" means "that/who will return" in English.
In this sentence, 'döneceği' is used as a relative clause indicating a future action related to the subject 'O'.
O, tatilden döneceği için çok heyecanlı.
He is very excited because he will return from vacation.
Here, 'döneceği' functions as a descriptor within a noun clause 'döneceği günü', specifying the day of return.
Döneceği günü bilmek istiyorum.
I want to know the day he will return.
In this case, 'döneceği' serves as part of an indirect statement, describing a possibility of action.