This is the 435th most frequent Czech word.
dubna
"Dubna" means "April" in English.
Here, 'dubna' is used in the context of a date, indicating the month in the genitive case.
První dubna je den, kdy si lidé dělají žerty.
The first of April is a day when people play jokes.
In this sentence, 'dubna' indicates a time frame within the month, again in the genitive case to denote 'during April'.
Během dubna se příroda začíná probouzet.
During April, nature begins to wake up.
Here, 'dubna' specifies a possessive relationship, 'of April', showing which days are being referred to, in genitive case.