There is not currently a way to have adTempus handle this automatically in a holiday set. Version 5 will be released in the next few months and it includes new settings to handle "observed" dates for holidays.

Until then, the way to handle such holidays is to create a separate rule set within the holiday definition for each year, and add the dates manually. In the main Holiday Set properties, there is a section at the bottom for "Rules for specific years". Add a new rule set in this section for a particular year, and then define the rules for that year.