While due dates for most executor tasks are calculated in relation to the decedent's date of death, certain tax-related tasks are instead due in relation to the calendar year ... and thus may occur early or late in the overall estate settlement process.
Decedent Income Tax Returns
The estate executor is responsible for filing final personal income tax returns for the decedent, typically by the normal US income tax due date of April 15 (in the calendar year following the date of death).
If the decedent had not yet filed tax returns for the year previous to his or her death (imagine that he or she died on January 15), then you are responsible for filing those previous year returns as well, by their normal due dates. This may mean you have very little time, or in some cases, that the deadline has already passed and penalties may already be owed.
Just as when doing your own taxes, you have the option to file for an extension, putting off the filing deadline for 6 months. See Filing for an Extension for more information of requesting a federal filing extension.
As usual, income tax forms need to be filed only if the decedent earned more than a minimum amount during the year, as set by statute.
See Paying Taxes for more information.
Annual Estate Income Tax Returns
An estate is considered to be a "taxpayer" in its own right, so it must file income taxes at the same time as everyone else (e.g., April 15 in the US).
As with the decedent income tax return mentioned above, you have the ability to file for a 6-month extension, and filing is only required if the estate earned more than a minimum amount during the year (count only the amount earned from the day after the date of death, until the end of the calendar year).
Note that you must do this every year until the estate is completely settled. Of course, in subsequent years, you count all the estate income generated during that entire calendar year.
Annual Property Taxes
If the estate contains real property, you must pay any relevant property taxes until the property is either sold or distributed to the heirs. Property taxes are often due in two installments during the calendar year; due dates vary widely by jurisdiction.
Annual Trust Income Tax Returns and Schedule K-1s
If you are also responsible for any estate-related trusts (as trustee), every year you must mail Federal Schedule K-1s to any trust beneficiaries by March 31 for any income distributions they received from the trust the preceding calendar year. If the trust retains over a certain minimum amount of undistributed income, then it will have to file its own tax forms (e.g., Federal Form 1041 and the corresponding state income tax form) by April 15.