Date of death estate valuation refers to the value of an asset or group of assets on the day that someone passes away. Date of death estate valuation is typically calculated using the "fair market value" of the asset at the time the person who owns it passes away. While it sounds simple enough, if you've ever sold something like an automobile, you know that fair market value is a subjective measurement, which is one reason that it's helpful to consult an experienced pro if you're trying to value your own estate or working as an estate administrator to handle tax matters.
In some specific cases, date of death might not even mean "the actual date the person passed away." That latter definition is used to value investment or retirement accounts, but stock market account values might be calculated using the average of prices on the days before and after the death if the person passed away on a day the markets were closed.
If you're looking to value personal effects such as art, jewelry or even business interests, then there's not typically a listed value for each item on the date of death. You would have to work with an appraiser to determine the fair market value of each item on that date.
To make things even more complex, you might opt to use an alternative valuation date. The Internal Revenue Service lets you choose to value the estate items based on their worth six months following the date of death. This provides you with two options so you can do the research and choose the valuation date that provides the most benefit to the estate or heirs.
Source: The Balance, "Do You Know How to Calculate the Value of Your Estate?," Julie Garber, accessed Jan. 20, 2017