The federal 941 is the primary federal employment tax form. Federal Withholding, Social Security, and Medicare Taxes which are withheld from employee payrolls are reported here and sent to the government. 941 forms are generally filed quarterly, along with the other State of Illinois payroll withholding and unemployment tax returns. They are due one month after the end of every quarter. If you are asking this question in the first place, then you obviously need a new accountant.