This is the 109th most frequent Turkish word.
dün
"Yesterday."
In this sentence, 'dün' is used as an adverb to indicate a specific time in the past (the day before today).
Dün, çok güzel bir dostumla kahve içtim.
Yesterday, I had coffee with a very good friend.
Here, 'dünün' is the possessive form of 'dün', used to describe something belonging to yesterday.
Dünün hava durumu çok güzeldi.
Yesterday's weather was very nice.
In this sentence, 'dün' is used in conjunction with a verb to provide emphasis on the time frame of the action, indicating 'yesterday'.