Depending on your level of expertise, it should be possible to do this with a Call Flow Designer (CFD) application. We did something similar with a "jobs line" IVR years ago. In our application, the user would dial the extension of the CFD which would instruct the user to record the new message, then this newly recorded message was be used to update the IVR prompt. This would likely require some C# code to update the holiday. It will not be simple.