Amazon's stock price slid on Friday, the last trading day of the year, after Trump tweeted that it's ripping off the US Postal Service. (vox.com)
This means you report gain or loss as if you sold it at the close of business on the last trading day of the year for its fair market value. (fairmark.com)
If you contact your broker on the last trading day of the year, you can complete a sale in the current year if your broker executes the trade that day. (fairmark.com)