Hmmm. IMO, I am not so sure that it does not work as a calendar. The thing to remember is that they are designed for kids, and for the most part - with my kids anyway - the joy of getting to open a new box each day is huge, and they won't care so much if it has a Christmas theme or not!
I can see both Merboy and Sirena just loving these two sets. And when they are done, they have additional parts they can use with their existing sets. While I liked the Christmas advent calendars, to me their usefulness was quite limited as soon as Christmas was over. (Besides, I already have more than a dozen Santa Clauses!
)
And, as Tiermann said, I will probably get an extra of the medieval calendar just for the parts...