This is the 977th most frequent Russian word.
четверг
Thursday.
The word 'четверг' is used here as a subject (noun) to indicate the current day of the week.
Сегодня четверг, и я иду в библиотеку.
Today is Thursday, and I am going to the library.
The word 'четверг' is used here to specify the temporal context of the scheduled meeting (prepositional case).
На четверг запланирована важная встреча.
An important meeting is scheduled for Thursday.
The word 'четверг' is part of a phrase indicating a habitual action occurring on this day of the week (accusative case with 'каждый').