I would like to broadcast a transaction. When should I "hold" and let the homestead release happen? Until when can I send transactions?
You won't need to "hold" or do anything special, as long as you're running a recently upgraded client that is Homestead ready (geth>1.3.5). As soon as we hit block number 1,150,000 the new protocol should become active, and we will be on the Homestead chain. As the end user, you should theoretically not be affected.
If you are accepting Ether as payment, I would consider requiring more confirmations during the rollout period, just in case there are any serious issues.