This is the 479th most frequent Hebrew word.
באחד
"In one" or "on one."
באחד is used here as 'on one of', referring to a singular instance within a group (days in this case).
פגשתי אותו באחד הימים הקרובים.
I met him on one of the upcoming days.
באחד is used here as 'in one of', referring to a single item within a specific group of items (books).
באחד מהספרים שקראתי מצאתי תשובה לשאלתי.
In one of the books I read, I found an answer to my question.
באחד is used here as 'on one of', describing a singular instance in a set (trips).