You need to appear in court, tell the judge that you are his wife and don't want to testify against him; and then you have to follow instruction of the judge. Your husband's lawyer should be able to give you guidance and help, as well. Your question is not complete as to all circumstances and factors that could come into play; but the initial advice above fits all, or almost all, situations. Good luck.
Talk to your husband's attorney about your assertion of the marital privilege. Except in certain limited circumstances, the state cannot force one spouse to be a witness against another, nor can they force you to reveal confidential communications between you and your husband.
I agree with the assessmants of my two colleagues, who have addressed all of the salient points. Given your interest in asserting the marital privilege, I suggest you reread their comments and follow up with their advice. Good luck.
This information is offered for educational purposes only. It is not intended as legal advice and you should not rely upon it to decide how to resolve this issue. No Attorney-Client relationship is intended or established by this response. You are faced with a situation where you need to consult with an experienced defense lawyer admitted to practice law in your State before you make any decisions as to how to resolve this issue.
I agree with my colleagues, with a few exceptions. Understand that there are TWO privileges at play - the marital testimonial privilege under Evidence Code section 970 (which says you have a privilege not to be called as a witness against your husband unless certain exceptions apply, which seem not to be relevant based upon what you state); and the marital communications privilege under Evidence Code section 980 (which provides that you and your husband both have a privilege to not disclose to anyone, again unless certain exceptions apply, the contents of any confidential communication made by one spouse to the other).
Thus, even if the judge makes you take the stand as a witness, you still may have a privilege to not disclose anything your husband told you in confidence.
The thing I might disagree with my colleagues on is that your husband's lawyer might have a conflict of interest in advising you as to whether to assert the marital testimonial privilege, since that privilege is YOURS to assert and NOT your husband's (your husband's attorney will certainly assert the marital communications privilege, since your husband is the holder of that privilege, and, accordingly, if the DA starts asking you about conversations between you and your husband, your husband's attorney should object, and that objection should be sustained by the judge). Although I cannot think of one based upon what you have disclosed, maybe there is a valid reason for you NOT to assert the testimonial privilege and to take the stand and tell the jury what you saw. Because your husband's lawyer is representing him, and your husband might not want you to testify for the DA, your husband's lawyer might bot be able to properly advise you.
If you have the means to do so, you might wish to consult your own independent counsel about this matter. If you do not have the means to do so, go to court on the subpoena date, and, when you arrive, if the DA or investigator wishes to speak to you in the hallway, as they often want to do, tell them you do not want to talk to them, but, if they call you as a witness, you will want to have the judge appoint a lawyer to represent you and advise you as to your rights since you are the defendant's spouse and you were married to the defendant on the date of the incident.
DISCLAIMER: This posting is not intended to constitute legal advice or to create an attorney-client relationship, but is for informational purposes only.