This is the 219th most frequent Czech word.
prvního
"Prvního" means "of the first" or "the first" in a genitive singular form, often used for dates (e.g., "on the first").
Here, 'prvního' indicates the day of the specified date, aligning with the genitive case used for dates in Czech.
V prvního května slavíme Svátek práce.
On May 1st, we celebrate Labor Day.
In this case, 'prvního' is used in the genitive masculine singular form to describe modification noun agreement.
To je nápad mého prvního učitele.
This is an idea from my first teacher.
Here, 'prvního dne' demonstrates the use of 'prvního' in the genitive case to agree with decree and to indicate the ordinal number.