Harold Prince, the production's producer and director, invited the critics to late previews for this production rather than having them all come to opening night. This was very unusual for a Broadway show at the time, but within a few years it became standard practice.
In addition, no performance was designated by Prince as the opening night. Ads for the production while it was in previews stated that previews would continue through November 17, 1968. Since reviews appeared on November 18, the performance on November 17 has generally been considered the production's opening night, rather than the preview that ads and ABC listings for the production labeled it.
In addtion, the production normally played a Monday through Saturday schedule, with evening performances starting at 8:30 p.m. (standard at the time). November 17 was a Sunday, and the curtain time was 7:30. This suggests that even though Prince advertised it as a preview, it was a special event and, for all intents and purposes, the unofficial or semi-official opening night.