This is the 313th most frequent Latvian word.
ceturtdien
"Thursday"
Here, 'ceturtdien' is used as an adverb to denote the specific day (Thursday) when the action (going to the market) will take place.
Es ceturtdien došos uz tirgu.
I will go to the market on Thursday.
In this sentence, the word form 'ceturtdienu' is a declension of 'ceturtdiena' (Thursday, the noun) used in the accusative case to indicate repetition or habitual action each Thursday.
Katru ceturtdienu mēs sanākam kopā ar draugiem.
Every Thursday, we gather with friends.
Here, 'ceturtdienai' is used in the dative case to express a deadline, the time by which an action (finishing the project) is expected to be completed.