This is the 679th most frequent Japanese word.
初日
"First day" or "opening day."
In this sentence, '初日' refers to the opening or first day of a movie's screening.
映画の初日はいつも混雑しています。
The opening day of the movie is always crowded.
Here, '初日' indicates the first day someone starts a new job.
今日は彼の仕事の初日です。
Today is his first day at work.
In this context, '初日' means the opening day of an event or exhibition.