menstrual period days; days of one's period
Refers specifically to the days when menstruation occurs. Often used in medical or personal health contexts.
I record my menstrual period days on a calendar.
生理日には体調が悪くなることが多い。
I often feel unwell on the days of my period.
Compound of 生理 (menstruation, physiology) and 日 (day).