The Czech word for day is
den

Translations for day and their definitions

den	©
1. n. day (24 hours, usually from midnight to midnight)
jednoho dne - one day, someday
Jednoho dne tě chytí. - They're gonna catch you one day.
po několika dnech - after a few days
za pár dní - in a couple of days
2. n. daytime (time between sunrise and sunset)
3. n. (astronomy) day (rotational period of a body orbiting a star)
Den na Merkuru trvá téměř 59 pozemských dní. - A day on Mercury lasts almost 59 terrestrial days.
4. n. genitive plural of dno
5. n. genitive plural of dna