Actually, when I called my sister in October to find out the date of the 1st Communion, it was so that I would know whether or not we could commit to the musical.
At our church, all the important CCD dates are given out at the beginning of the school year, so I knew in September when the exact dates were for each of my kids' 1st Communions. Also, at our church, you can choose to make your 1st Communion on the Saturday with all the other CCD classes or you can choose any other Mass during the Easter season. If you made your 1st Communion with all of the others, you were limited to only 10 attendees (including 1st Communicant's parents and sibings). If you made your 1st Communion at another Mass, you could invite as many people as you wanted. Both of my kids opted to make their 1st Communions on a different Sunday at our usual 9:30 Mass. I think it actually meant more for my kids to be congratulated by all the people we see in Mass every week and who remember when my kids were just infants.
It's entirely possible that my sister didn't know the exact date when I called and just figured that it was usually the first weekend in May and only now does she have the exact dates, but based on my experience, I would have thought she knew when it would be. I don't remember the exact conversation since that was almost 4 months ago now, but I would have thought that my sister would have asked about the concert dates before she contributed money toward the tickets.