Nothing has been announced, but if past practice holds, a new large-format Priority Mail stamp will be issued for this rate on Sunday, January 24, 2021, the date the rate goes into effect. There will not be a first day ceremony and in-person cancellations will be hard to obtain.
Updated December 26th:
Linn’s Stamp News reports in its January 11th edition (online December 26th) that this $7.95 stamp will feature the Castillo de San Marcos, located in St. Augustine, FL. It will be the 25th in the “American Landmark” series of “shipping rate” stamps.
From its official website, “Built by the Spanish in St. Augustine to defend Florida and the Atlantic trade route, Castillo de San Marcos National Monument preserves the oldest masonry fortification in the continental United States and interprets more than 450 years of cultural intersections.”
From this point on, the most recent entries are at the top.
Updated January 29th:
Technical Specifications:
Issue: Castillo de San Marcos Stamp
Item Number: 121300
Denomination & Type of Issue: $7.95 Priority Mail® Rate
Format: Pane of 4 (1 design)
Series: N/A
Issue Date & City: January 24, 2021, St. Augustine, FL 32084
Art Director: Greg Breeding, Charlottesville, VA
Designer: Greg Breeding, Charlottesville, VA
Artist: Dan Cosgrove. Chicago, IL
Modeler: Joseph Sheeran
Manufacturing Process: Offset, Microprint
Printer: Ashton Potter (USA) Ltd. (APU)
Press Type: Muller A76
Stamps per Pane: 4
Print Quantity: 5,500,000 stamps
Paper Type: Nonphosphored Type III, Block Tag
Adhesive Type: Pressure-sensitive
Colors: Black, Cyan, Magenta, Yellow
Stamp Orientation: Horizontal
Image Area (w x h): 1.42 x 1.085 in/36.068 x 27.559 mm
Overall Size (w x h): 1.56 x 1.225 in/39.624 x 31.115 mm
Full Pane Size (w x h): 4.12 x 3.45 in/104.648 x 87.63 mm
Plate Size: 168 stamps per revolution
Plate Number: “P” followed by four (4) single digits
Marginal Markings:
Front: Plate number in two corners of pane
Back: ©2020 USPS • USPS Logo • Barcode (121300) printed behind each stamp • Plate position diagram (7) • Promotional text
Updated January 6th:
Here is the Digital Color first-day Postmark for this issue:
It measures 2.68″ x 1.61″. The “standard killerbar” (“FIRST DAY OF ISSUE”) postmark will also be offered.
Note: Because of the high denomination, there is no minimum number requirement for cancellations submitted to Cancellation Services.
Updated December 31st from the Postal Bulletin:
On January 24, 2021, in St. Augustine, FL, the United States Postal Service® will issue the Castillo de San Marcos Priority Mail® stamp in one design, in a pressure-sensitive adhesive (PSA) pane of four stamps (Item 121300). This stamp will go on sale nationwide January 24, 2021, and must not be sold or canceled before the first-day-of-issue.
The latest Priority Mail stamp celebrates the oldest masonry fortification in the United States, the Castillo de San Marcos, in St. Augustine, Florida. The stamp art features a digital illustration of the fortress based on a contemporary photograph. With a view toward the northeast corner of the fortress, the artwork captures it in the golden glow of sunrise over Matanzas Bay. Art director Greg Breeding designed the stamp with art created by Dan Cosgrove.
No automatic distribution for tem 121300, $7.95 Castillo de San Marcos Priority Mail PSA Pane of Four
How to Order the First-Day-of-Issue Postmark:
Customers have 120 days to obtain the first-day-of-issue postmark by mail. They may purchase new stamps at their local Post Office™ or at The Postal Store® website at usps.com/shop. They must affix the stamps to envelopes of their choice, address the envelopes (to themselves or others), and place them in a larger envelope addressed to:
FDOI – Castillo de San Marcos Stamp
USPS Stamp Fulfillment Services
8300 NE Underground Drive, Suite 300
Kansas City, MO 64144-9900
After applying the first-day-of-issue postmark, the Postal Service™ will return the envelopes through the mail. There is no charge for the postmark up to a quantity of 50. There is a 5-cent charge for each additional postmark over 50. All orders must be postmarked by May 24, 2021.
Technical Specifications are not yet available. 
If you want a Single Stamp from this issue: Postmaster Oneco CT 06373 Mail-Orders Only ( No Internet/Telephone ) Check/M.O. payable to Postmaster Oneco CT $7.95 each No S/H fees – sent to You via Priority Mail .