(If due date falls on a weekend or holiday then the due date is the next business day)

Individuals / Sole Proprietors
Tax Returns: April 15th
Extensions: 6 Month extensions expire October 15th
Estimated Tax Payments:
1st:  April 15th
2nd: June 15th
3rd: September 15th
4th: January 15th (for previous tax year, i.e., pay 1/15/20 for 2019 tax year)

Partnerships
Tax Returns: March 15th
Extensions: 6 month extensions expire September 15th

C Corporations
Tax Returns: April 15th (Calender Year End) or on the 15th day of the 3rd month after Fiscal Year End
(i.e. Dec 15th for Sept 30 year end)
Extensions: 6 month extensions expire October 15th

S Corporations
Tax Returns: March 15th
Extensions: 6 month extensions expire September 15th

Payroll Tax Returns
Quarterly:
1st Qtr:  April 30th (for January-February-March)
2nd Qtr: July 31st (for April-May-June)
3rd Qtr: October 31st (for July-August-September)
4th Qtr: January 31st (for October-November-December)
Annual: January 31st

CA Sales Tax Returns
Quarterly:
1st Qtr:  April 30th (for January-February-March)
2nd Qtr: July 31st (for April-May-June)
3rd Qtr: October 31st (for July-August-September)
4th Qtr: January 31st (for October-November-December)
Annual: January 31st

Santa Clara County Business Property Returns
Annual: April 1st
May 7th last day to file without 10% penalty

1099-MISC & 1096
1099-MISC to Vendors: January 31st
1099-MISC and 1096 to IRS: January 31st