The Form 706 is the name of given to the Estate Tax return form. The due date for filing this form is usually nine months after the date of death of the taxpayer/decedent.
If you cannot file this tax return, not to worry, the IRS grants a six month extension if requested prior to the due date and the estimated correct amount of tax is paid before the due date.
The name of the extension form is Form 4768 and here is the IRS link to the extension form. http://www.irs.gov/pub/irs-pdf/f4768.pdf