Carol Lawrence was among the actresses seriously considered for the role of Lili.
Some online sources report that there were two previews, starting on April 12, 1961, which was a Wednesday. The show opened on the following day: Thursday, April 13.
There may have been only two previews, but they would have started on Tuesday, April 11, as we know there was a benefit preview that night for the scholarship fund of the Downtown Community School. This was reported on in the New York Times on March 31, 1961. It would not be unusual or surprising for there to be only one performance on Wednesday if it was the day before the opening so there may well have been only two previews, but they probably started on Tuesday of that week, not Wednesay.