This is the 64th most frequent Greek word.
την
"The" (feminine singular accusative in Greek).
Here, 'την' is the definite article used to introduce the direct object 'τσάντα' (bag), which is feminine singular in accusative case.
Βρήκα την τσάντα σου.
I found your bag.
In this sentence, 'την' functions as the definite article modifying the object 'ημέρα' (day), which is again feminine singular in accusative case, indicating a specific day.
Θυμάμαι την ημέρα εκείνη.
I remember that day.
Here, 'την' acts as a pronoun replacing the direct object, referring to a feminine singular noun previously mentioned in conversation.