From the U.S. Postal Service announcement on October 22, 2019:

With this commemorative stamp, the Postal Service marks the 400th anniversary of the arrival of the Mayflower in Plymouth Harbor. The stamp’s image is based on artwork of watercolor, acrylic and gouache, a method of painting that uses opaque pigments ground in water and thickened to a glue-like consistency. The painting was digitally refined to convey a scene of desolate beauty at the end of the Pilgrims’ long journey to an unfamiliar world. Art director Greg Breeding designed the stamp with original art by Greg Harlin.

This stamp will be issued Thursday, September 17th in Plymouth, MA.

The Scott catalogue number for this issue is 5524.

Additional information will be posted below the line, with the most recent at the top.

Updated August 18th:
Here are the first-day postmarks for this issue: (Measurements corrected on the 19th)The Digital Color Postmark measures 2.69″ x 1.46″ (Corrected August 19th)
The pictorial postmark measures 2.70″ x 1.49″. (Corrected August 19th.)

Updated August 14th from the Postal Bulletin
On September 17, 2020, in Plymouth, MA, the United States Postal Service® will issue the Mayflower in Plymouth Harbor stamp (Forever® priced at the First-Class Mail® rate) in one design, in a pressure-sensitive adhesive pane of 20 stamps (Item 476700). The stamp will go on sale nationwide September 17, 2020, and must not be sold or canceled before the first-day-of-issue. The Mayflower in Plymouth Harbor pane of 20 stamps may not be split and the stamps may not be sold individually.

With this commemorative stamp, the United States Postal Service marks the 400th anniversary of the arrival of the Mayflower in Plymouth Harbor. The watercolor, gouache, and acrylic painting was digitally refined to convey a scene of desolate beauty at the end of the Pilgrims‘ harrowing journey to an unfamiliar world. The stamp also features a stylized hawthorn flower printed in intaglio. In England, the hawthorn — a member of the rose family — is sometimes called a mayflower, as it blooms in May. Art director Greg Breeding designed the stamp with original art by Greg Harlin.

No automatic distribution of Item 476700, Mayflower in Plymouth Harbor

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 – Mayflower in Plymouth Harbor 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 January 17, 2021.

Technical Specifications:

Issue: Mayflower in Plymouth Harbor Stamp
Item Number: 476700
Denomination & Type of Issue: First-Class Mail Forever
Format: Pane of 20 (1 design)
Series: N/A
Issue Date & City: September 17, 2020, Plymouth, MA 02360
Art Director: Greg Breeding, Charlottesville, VA
Designer: Greg Breeding, Charlottesville, VA
Artist: Greg Harlin, Annapolis, MD
Modeler: Joseph Sheeran
Manufacturing Process: Offset, Intaglio, Microprint
Printer: Ashton Potter (USA) Ltd. (APU)
Press Type: Stevens Vari-Size Security Press
Stamps per Pane: 20
Print Quantity: 25,000,000 stamps
Paper Type: Nonphosphored Type III, Block Tagged
Adhesive Type: Pressure-sensitive
Processed at: Ashton Potter (USA) Ltd. (APU)
Stamp Orientation: Horizontal
Image Area (w x h): 1.42 x 0.84 in./36.068 x 21.336 mm
Stamp Size (w x h): 1.56 x 0.98 in./39.624 x 24.892 mm
Full Pane Size (w x h): 10 x 6.35 in./254 x 161.29 mm
Press Sheet Size (w x h): 20.125 x 12.825 in./511.175 x 325.755 mm
Colors: Black, Cyan, Magenta, Yellow, Intaglio Black
Plate Size: 80 stamps per revolution
Plate Number: “P” followed by five (5) single digits
Marginal Markings:
Front: Header: MAYFLOWER IN PLYMOUTH HARBOR • Plate number in bottom two corners
Back: ©2019 USPS • USPS logo • Two barcodes (476700) • Promotional text • Plate position diagram (4)

Updated February 18th:
In an interview with Wicked Local/Plymouth (a website owned by Gannett Media) shortly after this stamp was announced, organizers of the Plymouth 400 celebration expressed disappointment about the stamp design.

“We have been asking that any stamps they produce include not only the European side of the story, but also the Native American perspective,” said George McKay, a Plymouth 400 Inc. volunteer who headed the philatelic effort. “Unfortunately for us, they chose to only produce one stamp for 2020, which does not include the other side of the story.”

Plymouth 400 is also running a FDC cachet contest for this issue, although there is no mention of what the prizes or compensation might be. You can find out how to enter here. The deadline for submissions is June 30th.