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.
I plan to go on the first day of the movie.
Many people came from the first day.
Also used for the first day of a new job, school term, or personal endeavor.
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.
There is a meeting on the first day of next month.
Specifically New Year's Day.
The first day of 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).
We hand out textbooks on the first day of the semester.
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.
Today is the first day of work (after the New Year holiday).
First day of attendance at a new job. More formal.
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.
April 1st is April Fools' Day.
元日は祝日です。
New Year's Day is a national holiday.
The first day ended up being just travel.