請問西鐵全月通6月份的首天生效時間為6月1日首班車開始? 還是以6月1日淩晨00:00(5月31日24:00)開始?

First Train.

For all relevant purposes, many public transportation do not define a day based on the normal date and time system. Instead, they define "day" based on the service intervals of the day, i.e. First and Last service.

但港鐵職員答案係話6月1日首班車開始計算

計日期，六月一日12點後入閘