elksong, good advice. You just never really know.
That first week can be incredible, as far as response. Sometimes they get going alone then, but in general it is in response. You can really get a group of bulls fired up though, then they feed off each other.
The biggest bull we took last year came into me screamin his fool head off, on the 4th. The year before that on opening day we had them going really good right at morning, etc... SO, you never know. Before the bigger bulls have a bunch of cows to get herded up, get in there and call him up. He'll be responsive as he's been waiting for this for a month. Rattling also works VERY good that week. Start slow and create excitement and the bulls will respond.
Speaking of hard horned, I caught a bull rubbing on wednesday! The time is near.