This is the 98th most frequent Korean word.
첫
"첫" means "first" in English.
The word "첫" indicates the ordinal concept of "the first" in connection to the noun "수업날" meaning "class day".
저는 첫 수업날에 늦지 않았습니다.
I was not late on the first day of class.
Here, "첫" is used to describe "사랑" (love), denoting the initial or first experience of a specific sentiment.
그는 그의 첫 사랑을 절대 잊지 못할 것입니다.
He will never forget his first love.
In this context, "첫 번째" (a variation using "첫" with "번째" enhancing the ordinal nuance) specifies the inaugural occurrence of an event, the "trip".