Hi there,
I like to do heads ten days before the date of the show, and for winter hair, clip on body hair once a week for three weeks and touch-ups at the show the day before they head into the ring. Of course, I always miss a bit, some pops up as I work the hair and it grows and some pops up when they hit the soft water at whatever city the show ends up being in. A very good fitter once said that the longer you can leave between the roughing out and the final clipping, the better. He said, if you clip early, you can let the hair grow back and cover your mistakes - and if you're not sure quite how you're going to do the job - it gives you time to turn them out in the field and 'sleep on it' until the inspiration comes to you (because no two clip jobs are ever the same - always tailored to the build of the animal).
As far as the halterbreaking - if you've got something that learns fairly easy - its definately possible. However, I would skip this show in favor of making sure the calf is ready for the next one. I like to have a minimum of three weeks of work into them before they hit the road. If something happened to really scare your calf at the show because you hadn't taken the time to properly condition it to all the sights and sounds it needs to be used to, it will NEVER be the same again and you won't be taking it to any more shows. Furthermore, you have to think about safety. You might think its safe for you to show a partially broken calf, but what if it got away and hurt some nine-year old child. To me, its not worth the risk. There are always more shows to go to down the road.