noun
first day; day one
Used for the first day of an event, period, or new situation such as work or school.
新学期の一日目は緊張する。
I get nervous on the first day of the new term.
旅行の一日目は雨だった。
The first day of the trip was rainy.
初日 often refers to the opening day of an event or performance, while 一日目 is a more general 'first day' counter.
Compound of 一日 (いちにち, 'one day') and the ordinal suffix 目 (め, '-th').