I don't understand why they can't announce a set schedule! Especially with playing up the fact they have a particular actress in the role normally.
When I saw the original production of Miss Saigon May of 1994 the Kim Alternate had a set schedule - Wed. matinees, Thursday evenings, and Saturday matinees according to the Playbill. However I saw the show on a Monday night and saw the alternate go on (there was an understudy slip to notify, of course).
The next time I saw the show was October of 1996 and the alternate schedule was Wed. evenings and Saturday matinees.
September of 1997 was still Wed. eve. and Sat. mat.
Finally, May 1998, it remained Wed. eve. and Sat. mat.
After that any time I was at the Broadway for Saigon I was ushering and didn't keep Playbills from that. But I don't remember ever looking at a Playbill for the show that just said "at certain performances" like is listed in the current Playbill (at least according to the listing on Playbill.com.)
The Christine Alternate in Phantom also ALWAYS had specific performance days as well (back then). My first time at Phantom (March of 1993) it was listed as Monday and Wednesday evenings. I won't go through that whole list as it would be MUCH longer. LOL! Just strange they wouldn't realize people would want to know, but then again Macintosh has ALWAYS been about the SHOW being the star and not the actors in it!!!
<-- Tevye, FIDDLER ON THE ROOF, March 2018