Employer or workers' compensation insurer?
The only requirement under state law is that your employer needs to pay you for the hours you actually worked. If your employer does pay you for the hours you worked on the day your got hurt, then that day would be considered the first date of disability. If your employer paid you for the entire day, or shift, then the next day would be considered the first day of disability and would be paid by the insurance company provided you are out for 21 or more full or partial calendar days.
If you are out for less than 21 full or partial calendar days, then you would not get paid. Under the Massachusetts Workers' Compensation law, you don't get paid for the first 5 full or partial calendar days unless you are unable to work for 21 or more full or partial calendar days.