The December 12th USPS Postal Bulletin indicates that this issue will be released April 25th in New York City. The April 17th Bulletin just has “October.”

From my article from the October press preview:

Details for the Pioneers of Graphic Design sheet of 12 stamps are still not set, since there are rights issues. The sheet will commemorate the 100th anniversary of the American Institute of Graphic Arts, and the first day ceremony will be tied into an AIGA event, probably in New York City in April. April. One of the designers whose work was shown was Bradbury Thompson, a major stamp designer of the mid-20th century (although, surprisingly, Wikipedia doesn’t mention that). The consensus among the reporters present was that Thompson should be a lock for whatever the final choices are.