It depends! If you are a new

H-1B beneficiary, you can start to work on 10/01/05. But if you are a transfer H-1B (from one employer to the other), you can work at the time of filing H-1B petition. You do not even need to wait till approval. Usually, H-1B approval notice is sent to the employer. Your employer is the petitioner, you are only the beneficiary. Check with your employer who is the petitioner.