The first day of an event or period (e.g., first day at work, the opening day); day one.
新しい職場の一日目は緊張してよく眠れなかった。
I was so nervous on the first day at the new job that I couldn't sleep well.
旅行の一日目は市内観光に充てる予定です。
We plan to spend the first day of the trip sightseeing in the city.
公演は三日間で、一日目に特別ゲストが登場した。
The performance runs for three days, and a special guest appeared on the first day.
研修の一日目はオリエンテーションが中心だった。
The first day of the training was mainly orientation.