"first postoperative day" - meaning

At first glance, the day of surgery shouldn't be called a post-operative day. Rather, it should be the next day. There is a paragraph in the source article that supports this.

Anesthesiology "Pain intensity on the first day after surgery"

Well-down the page this text appears:

The personnel were trained to collect data in a standardized manner and were not part of the responsible surgical or anesthesia team. To reduce selection bias, data collection took place on randomly selected days. These dates were not known in advance to the medical staff, and on a survey day all patients who had been operated on the day before were considered for inclusion.

That makes it clear that by first post-operative day, they mean the day after surgery.