Uhh, that's not how it actually works. Now, if you want to limit the tax break to some set of things the government actually does anyway then I think you would be hard pressed to find donors unless they did more than the government. At which point your only limiting it to the same kinds of things (ex: new roads) not the exact same things.
If the government would otherwise be financing the festival, then yes, otherwise no tax credit, and IMHO, no tax break, should be given.