On the first day of employment, under normal circumstances, the relevant procedures of the company have not been completed, and it is too late to sign the contract. According to the law, it is legal to conclude a labor contract within one month from the date of employment. Therefore, it is not the first day of employment, but still in the probation period, and the contract must be signed immediately. As long as it is within one month from the date of employment, both parties can sign a written labor contract.