lesson day; practice day
Refers to a scheduled day for a lesson or practice, especially in traditional arts, martial arts, or music.
Please write down next week's lesson day on the calendar.
Today is the day for my tea ceremony lesson.
練習日 is a more general term for 'practice day' used in sports or music, while 稽古日 often implies traditional arts or disciplined training.
Compound of 稽古 (keiko, 'practice, lesson') and 日 (bi, 'day'). The reading けいこび follows rendaku voicing of ひ to び.