September 5
Employers. Semi-weekly depositors must deposit employment taxes for payroll dates August 28-30.

September 6
Employers. Semi-weekly depositors must deposit employment taxes for payroll dates August 31-September 3.

September 10
Employees who work for tips. Employees who received $20 or more in tips during August must report them to their employer using Form 4070.

September 11
Employers. Semi-weekly depositors must deposit employment taxes for payroll dates September 4-6.

September 13
Employers. Semi-weekly depositors must deposit employment taxes for payroll date September 7-10.

September 16
Individuals. Individuals who do not pay tax through withholding deposit the third installment of estimated tax for 2013.
Corporations. Corporations deposit the third installment of estimated tax for 2013.
Corporations. Corporations and S corporations that obtained 6-month extensions file their 2012 Form 1120 or 1120S and pay tax due.
Partnerships. Partnerships that obtained 5-month extensions file their 2012 Form 1065.

September 18
Employers. Semi-weekly depositors must deposit employment taxes for payroll dates September 11-13.

September 20
Employers. Semi-weekly depositors must deposit employment taxes for payroll dates September 14-17.

September 25
Employers. Semi-weekly depositors must deposit employment taxes for payroll dates September 18-20.

September 27
Employers. Semi-weekly depositors must deposit employment taxes for payroll dates September 21-24.

October 2
Employers. Semi-weekly depositors must deposit employment taxes for payroll dates September 25-27.

October 4
Employers. Semi-weekly depositors must deposit employment taxes for payroll dates September 28-30.