You're unlikely to be in the mood to deal with the tax implications of losing your legal partner soon after his or her passing. Unfortunately, the tax issues surrounding the "separation by death" of two spouses must be addressed beginning in the tax year in which the death occurs. In other words, you're likely to have to make significant changes to your tax strategy and filing practices within a year of your spouse's passing.
Since this situation occurs countless times per year, there is a robust body of legal precedent to govern it. You can handle your filing for the calendar year during which your spouse passed on in one of several ways. Your ultimate choice will depend upon your current financial situation, your long-term financial goals, and your marital status at the end of the year in which your spouse died.
In most cases, you'll be permitted to file your taxes jointly with your deceased spouse for their death year. You'll need to glean accurate income information from their records to avoid an IRS audit. If your spouse was employed at the time of his or her death, you may need to contact their old employer to procure a W-2 form. You may also need to talk to the executor of your spouse's estate to determine what other forms of income or deductions should be included in your spouse's final tax return.
If you remarry before the end of the year in which your spouse dies, you can't file joint taxes with either partner for that year. You'll need to file your own taxes as an unattached taxpayer. You'll also need to file your deceased spouse's taxes using the "Married Filing Separately" protocol. Your new spouse would be required to file his or her taxes according to this protocol as well.
If you don't remarry before the year is out, you can still file your taxes as a single taxpayer. Since this option rarely produces any financial advantages, most taxpayers ignore it.
In the future, you may be able to qualify for lower tax rates by filing your taxes as a "Qualifying Widow." If you have dependent children in your household, you can claim widow or widower status for two tax years after your spouse passes on. However, there are no circumstances in which you can claim this status for the tax year in which your spouse actually dies.