There are various versions of the 12 Letters of Christmas (that's what I call it, the actual title seems to vary a bit making it hard to search).

Here is one (click me)...

And another one (click me)...

The second uses more restrained language, suitable for a family audience.

To search, I found the best combo of required words to be:
letters christmas agnes "must insist" sanitarium

The series of letters works well as a thread through the whole concert, read in clusters of 2-3, or they can be read all at once during a musicians' break.