All new employees must complete Section 1 of Form I-9 on or before their first day of employment. Then, within three business days of their start date, they should submit acceptable proof of their identity and eligibility to work in the United States.
As the employer, you should complete Section 2 within those same three business days. If the duration of the job will be fewer than three days, you should complete Section 2 no later than the first day of employment. Section 2 is generally done at the time the employee brings in their identifying documents, as it asks for specific information about these forms of identification.