This is the 734th most frequent Czech word.
deník
Diary; journal; newspaper.
Here, 'deník' is used to mean a personal diary where someone records their daily events or thoughts.
Každý večer píšu do svého deníku.
Every evening, I write in my diary.
In this usage, 'deník' refers to a newspaper, a printed daily publication with news and articles.
Tento deník obvykle informuje o aktuálních událostech.
This newspaper usually informs about current events.
The word 'deník' in this context is interpreted as a journal or periodical publication read by an audience.