This is the 48th most frequent Hebrew word.
הראשון
"The first."
In this sentence, הראשון ('the first') is used as an ordinal adjective to describe 'book' (ספר), referring to its order in a sequence, specifically the initial one I read.
הספר הראשון שקראתי היה מלא הפתעות.
The first book I read was full of surprises.
Here, הראשון ('the first') modifies 'day' (יום) to denote the inaugural day following the start of the vacation period.
ביום הראשון של החופשה ירד גשם.
On the first day of the vacation, it rained.
In this example, הראשון ('the first') serves as a nominal identifier representing a person who initiates the action of getting up in the specified time frame.