1st April 15
2nd June 15
3rd September 15
4th December 15
1st 15th day of 4th month of the taxable year
2nd 15th day of 6th month of the taxable year
3rd 15th day of 9th month of the taxable year
4th 15th day of 12th month of the taxable year
26 C.F.R. §1.6655-1