For better or worse, you "are" the case against this guy. Police confession or no police confession, you are 100% a necessary witness for a host of reasons. In order to assist, may I ask a few questions:
1. Exactly what is the case number?
2. Exactly what is the case name (People vs. who?)
3. Exactly what Department or Division are you supposed to go to?
4. Have you been in contact with the City Attorney or District Attorney?
5. Have you actually been personally served with an actual subpoena/notice to appear?
6. Is this appearance for a trial or a "preliminary hearing" (or does the subpoena even say?)
I could write on and on about the various possibilities of what could happen to any criminal case between now and December 26th, but what I'd rather do is have you give me the information and, if you'd like, I could make sure that the court will permit you to be "on call". I'm a big believer in civic duty and doing what's right for our common good, but there nevertheless are things that can be done to accommodate witnesses like yourself.
If you'd rather handle this as a P.M., that's fine too. I happen to be a lawyer so I have access to people, phone numbers and stuff that might be useful. Feel free to contact me.