noun
admission date; first day
Used for the day someone enters a facility such as a training institute, care home, or prison. Often appears in official contexts.
入所日は来週の月曜日です。
The admission date is next Monday.
入所日に必要な書類を忘れずに持ってきてください。
Please remember to bring the necessary documents on the first day.
入学日 is for school enrollment; 入所日 is for non-school facilities like training centers or care homes.
Compound of 入所 (entering a facility) + 日 (day).