This is the 435th most frequent Czech word.
dubna
"Dubna" means "April" in English.
Here, 'dubna' specifies a possessive relationship, 'of April', showing which days are being referred to, in genitive case.
Některé dny dubna přinesly krásné počasí.
Some days of April brought beautiful weather.
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'.