It has been a very long time (many years) since I looked at the calendar.sty code. While in principle I would be very happy to help, and I'm glad to see that the code is still finding use in the wild, I don't have available time for coding on the project at present.
The best I can do is make the source available in a versioned store, so that people can work with it collaboratively to fix bugs, address limitations, or add extensions. I've put it up on the BitBucket service, which uses the Mercurial version control system:
(this forum apparently does not permit links, but you can use a search engine to find the source, with keywords such as "latex calendar bitbucket")
Feel free to help yourself to the archive, either for download or for cloning, and (to everyone) let me know if you'd like write access to the project.
Sorry I can't be of more assistance,
Frank Bennett