The earned sick time law applies to all employees, no matter how many hours they work. At a minimum, part time employees earn one hour of sick time for every 30 hours they work, though your employer may opt to simply give everyone 40 hours, or five days, per year.
This means if you work 20 hours per week for 50 weeks in a year, you will acrue 33.3 hours of sick time on the accrual method.
​Though you are entitled to accrue the sick time beginning with your first day of work, your employer is allowed to delay your use of that time for the first 90 days of your employment.
Learn more here about the Massachusetts earned sick time law.