From the U.S. Postal Service announcement on November 17, 2020:
From the USPS: In recent decades, Day of the Dead has caught on in the United States as a festive and increasingly popular celebration for all ages. These new stamps will be the first issued by the U.S. Postal Service to mark this holiday. Luis Fitch designed and illustrated the stamps. Antonio Alcalá was the art director.
These stamps will be issued September 30th with a ceremony in El Paso, Texas.
Here are the Scott catalogue numbers:
5640 Girl’s Skull with Bow
a. Imperforate
5641 Man’s Skull with Hat
a. Imperforate
5642 Woman’s Skull with Curled Hair
a. Imperforate
5643 Boy’s Skull
a. Imperforate
b. Horiz. strip of 4, #5640-5643
c. Imperforate strip of 4, #5640a-5643a
Additional information will be posted below the line, with the most recent at the top.
Updated October 21st:
Here is the Digital Color Postmark for this issue:
It measures 2.94″ x 1.37″. There is no pictorial black-and-white postmark for this issue, just the FIRST DAY OF ISSUE postmark.
Updated August 28th from the Postal Bulletin:
On September 30, 2021, in El Paso, TX, the United States Postal Service® will issue the 
Day of the Dead stamps (Forever® priced at the First-Class Mail® rate) in four designs, in a pressure-sensitive adhesive (PSA) pane of 20 stamps (Item 481400). These stamps will go on sale nationwide September 30, 2021, and must not be sold or canceled before the first-day-of-issue. The Day of the Dead pane of 20 stamps may not be split and the stamps may not be sold individually.
With the release of four colorful new stamps, the Postal Service™ celebrates Day of the Dead, an increasingly popular holiday in the United States. The stamp art features several iconic elements of a traditional Day of the Dead ofrenda, or offering, which honors departed relatives as well as heroes. Stylized, decorated “sugar skulls” are personalized as four family members, one per stamp. The lit candles flanking each sugar skull are beacons 
to guide deceased loved ones on their annual return journey to the land of the living. Dotting each stamp and embellishing their shared vertical borders are marigolds (cempazuchitles), the most popular Day of the Dead flower. Luis Fitch designed and illustrated the stamps. Antonio Alcalá was the art director.
No automatic distribution.
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 – Day of the Dead Stamps
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 30, 2022.
Technical Specifications:
Issue: Day of the Dead Stamps
Item Number: 481400
Denomination & Type of Issue: First-Class Mail Forever
Format: Pane of 20 (4 designs)
Series: N/A
Issue Date & City: September 30, 2021, El Paso, TX 79910
Art Director: Antonio Alcalá, Alexandria, VA
Designer: Luis Fitch, Minneapolis, MN
Artist: Luis Fitch, Minneapolis, MN
Modeler: Sandra Lane/Michelle Finn
Manufacturing Process: Offset
Printer: Banknote Corporation of America
Press Type: Alprinta
Stamps per Pane: 20
Print Quantity: 35,000,000 stamps
Paper Type: Phosphor, Overall
Adhesive Type: Pressure-sensitive
Colors: PMS 185 C, PMS 191 C, PMS 3262 C, PMS 376 C, PMS 021 C, PMS Purple C, PMS 213 C, PMS Yellow C, PMS Black 6 C
Stamp Orientation: Horizontal
Image Area (w x h): 1.05 x 0.77 in./ 26.67 x 19.558 mm
Overall Size (w x h): 1.19 x 0.91 in./30.226 x 23.114 mm
Full Pane Size (w x h): 5.76 x 5.55 in./146.304 x 140.97 mm
Press Sheets Size (w x h): 23.04 x 11.1 in./585.216 x 281.94 mm
Plate Size: 320 stamps per revolution
Plate Number: “B” followed by nine (9) single digits in four corners
Marginal Markings:
Front: Plate number in four corners
Back: ©2020 USPS • USPS logo • 2 barcodes (481400) • Plate position diagram (8) • Promotional text
