This is the 534th most frequent Croatian word.
prvih
"Prvih" means "the first" (genitive plural form) in English.
'Prvih' is used here to indicate the ordinal number of a group ('first') combined with the definite article within the genitive plural form of the noun.
Pročitao sam prvih deset stranica knjige.
I have read the first ten pages of the book.
'Prvih' functions as 'first' modifying 'several days,' and is placed in the genitive plural form to agree with the quantified term.
Prvih nekoliko dana na poslu može biti izazovno.
The first few days at work can be challenging.
'Prvih' denotes 'first' applying to the phrase 'hour of time,' and matches the genitive form to agree with the time expression.