If you want to create a tax loss the sale has to be completed before the last day of the year. That is ok if you have no intention of repurchasing. If you want to get back in it may be wiser to create your tax loss 30 days before year end so that you can have the liberty to execute the repurchase as early as possible in the new year. The entire month of dec is open for tax selling do not assume that it all takes place at the last possible moment. A tax loss can be created at any time as long as you are not back in within 30 days.