Translation guide
In Japanese, expressing "first day" depends on what it is the first day of. The most common and versatile word is 初日 (shonichi), used for events, performances, and new endeavors. For calendar dates, use 1日 (tsuitachi), which specifically means the first day of the month. Other expressions exist for first day of the year, first day of work, etc.
The first day of something like a festival, theater run, job, school, or any scheduled event.
The most common and versatile word for 'first day' of an event, performance, exhibition, or new undertaking. It emphasizes the opening day.
映画の初日に行く予定です。
I plan to go on the first day of the movie.
初日からたくさんの人が来ました。
Many people came from the first day.
Also used for the first day of a new job, school term, or personal endeavor.
新しい仕事の初日は緊張します。
I get nervous on the first day of a new job.
Referring to the 1st day of a calendar month.
The standard word for the first day of the month. Note the irregular reading.
来月の1日に会議があります。
There is a meeting on the first day of next month.
4月1日はエイプリルフールです。
April 1st is April Fools' Day.
Specifically New Year's Day.
Refers specifically to the morning of January 1st, but often used for the whole day.
元旦に初詣に行きます。
I go to the first shrine visit of the year on New Year's Day.
Refers to January 1st as a date, often used in formal contexts.
The first day of travel.
Simply 'first day of the trip'. 初日 is combined with 旅行 (trip).
旅行の初日は雨でした。
The first day of the trip was rainy.
Can be used alone if context makes it clear you're talking about a trip.
初日は移動だけで終わりました。
The first day ended up being just travel.
The beginning day of a defined period like a semester, season, or era.
Attach の初日 to the period word. E.g., 学期の初日 (first day of the semester), 夏休みの初日 (first day of summer vacation).
学期の初日に教科書を配ります。
We hand out textbooks on the first day of the semester.
夏休みの初日は海に行った。
On the first day of summer vacation, I went to the beach.
The first day at a new job or the first working day of the year.
Specifically the first working day of the new year, or the first day back at work after a break. Also used for the first day of a new job.
今日は仕事始めです。
Today is the first day of work (after the New Year holiday).
First day of attendance at a new job. More formal.
初出勤の日はスーツを着ます。
On the first day of work, I wear a suit.
初日 (shonichi) is not used for the first day of the month. Use 1日 (tsuitachi) instead. Saying 初日 for a date sounds unnatural.
初日 is for events, performances, and new phases. 1日 is strictly for the first day of the month. For the first day of the year, use 元旦 or 元日.
元日は祝日です。
New Year's Day is a national holiday.