The Scott catalogue number for this issue is 5284.
Latest updates at the bottom.
From the USPS:
Flag Act of 1818 
With this stamp, the Postal Service marks the 200th anniversary of the Flag Act of 1818, which gave us the basic design of the current American flag: 13 stripes symbolizing the original 13 colonies and one star for each state in the union.
This stamp will be issued June 9 in Appleton, Wisconsin, which holds a large Flag Day Parade each year.This year’s parade will be held June 9th, the same day the stamp will be issued.
The Flag Act of 1818 was enacted by Congress on April 4, 1818. For more on this Act, visit Wikipedia.
Appleton was the first-day city for the Edna Ferber stamp (Sc. 3433) in 2002. It is also the birthplace of Harry Houdini (Sc. 3651), although that stamp was issued in New York.
—VSC
Updated May 1st:
[press release]
Celebrating the 200th Anniversary of the Flag Act of 1818
U.S. Postal Service Dedicating a New Forever Stamp Marking the Historic Milestone
What: With this stamp, the U.S. Postal Service is pleased to highlight the 200th anniversary of the Flag Act of 1818, which provided the basic design of the current American flag: 13 stripes symbolizing the original 13 colonies and one star for each state in the union.
The stamp art celebrating this anniversary shows a flag with 20 stars, the number of states in the Union when the Flag Act of 1818 went into effect. Ethel Kessler was art director for the stamp and typography was by Kit Hinrichs. Hinrichs shows a flag with crisp folds and a layering effect that conveys a sense of the dynamism of the young nation. News of the stamp is being shared with the hashtags #FlagActStamp.
Who:
Pat Mendonca, U.S. Postal Service Senior Director for the Postmaster General/CEO
When:
Saturday, June 9, 2018, 11:30 a.m., Central Daylight Time
Where: 68th Appleton Flag Day Parade
Houdini Plaza
100 W. Lawrence Street
Appleton, WI 54911
Followers of the U.S. Postal Service’s Facebook page can view the ceremony live at facebook.com/USPS.
RSVP: Dedication ceremony attendees are encouraged to RSVP at usps.com/flagact to gain entrance to the event.
Background: Two hundred years ago the Flag Act of 1818 gave us the basic design of the current American flag: 13 stripes and one star for each state in the Union.
Prior to the act, the nation’s official flag showed 15 stars and 15 stripes. The expansion of the union to 20 states by 1817 required a rethinking of the flag’s configuration. Rather than increasing the number of stripes every time a new state joined the union, the Flag Act reduced their number to 13, signifying the original 13 colonies, and increased the number of stars to reflect the current number of states in the union. The act specified that a new star would be added on the Fourth of July following the admission of a new state.
The United States flag has had 50 stars since July 4, 1960, when a new star was added after Hawaii became the 50th state.
Updated May 10th: Here are the first-day cancels for this issue: 
The DCP measures 2.34″ x 1.49″ 
The pictorial cancel measures 2.91″ x 1.47″
From the Postal Bulletin:
On June 9, 2018, in Appleton, WI, the U.S. Postal Service® will issue the Flag Act of 1818 stamp (Forever® priced at the First-Class Mail® rate) in one design, in a pressure-sensitive adhesive pane of 20 stamps (Item 477800). The stamp will go on sale nationwide June 9, 2018.
With this stamp, the Postal Service™ marks the 200th anniversary of the Flag Act of 1818, which provided the basic design of the current American flag: 13 stripes symbolizing the original 13 colonies and one star for each state in the Union. The stamp art features a flag with 20 stars, the number of states in the Union when the Flag Act of 1818 was implemented. The flag‘s crisp folds and layering effect convey a sense of the dynamism of the young nation. Ethel Kessler served as art director for the project with stamp design and typography by Kit Hinrichs.
Stamp Fulfillment Services will not make an automatic push distribution to Post Offices.
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 – Flag Act of 1818 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 October 9, 2018.
Technical Specifications:
Issue: Flag Act of 1818 Stamp
Item Number: 477800
Denomination &T ype of Issue: First-Class Mail Forever
Format: Pane of 20 (1 design)
Series: N/A
Issue Date & City: June 9, 2018, Appleton, WI 54911
Art Director: Ethel Kessler, Bethesda, MD
Designer: Kit Hinrichs, San Francisco, CA
Typographer: Kit Hinrichs, San Francisco, CA
Modeler: Joseph Sheeran
Manufacturing Process: Offset
Printer: Ashton Potter (USA) Ltd. (APU)
Printed at: Williamsville, NY
Press Type: Offset, Microprint
Stamps per Pane: 20
Print Quantity: 20,000,000 stamps
Paper Type: Nonphosphored Type III, Block Tag
Adhesive Type: Pressure-sensitive
Processed at: Ashton Potter (USA) Ltd. (APU)
Colors: Black, PMS 2145C Blue, PMS 186C Red, and PMS Cool Gray 6C
Stamp Orientation: Horizontal
Image Area (w x h): 1.42 x 0.84 in./36.07 x 21.34 mm
Overall Size (w x h): 1.56 x 0.98 in./39.62 x 24.89 mm
Full Pane Size (w x h): 7.24 x 5.92 in./183.90 x 150.37 mm
Plate Size: 240 stamps per revolution
Plate Numbers: “P” followed by four (4) digits
Marginal Markings:
Front: Plate numbers in two corners
Back: ©2017 USPS • USPS Logo • Two barcodes (477800) • Plate Position Diagram • Promotional Text
WHO:
It appears the actual nametag, at least for this flight, read just “Sally.”
On April 6, 2018, in Washington, DC, the U.S. Postal Service ® will issue the STEM Education stamps (Forever ® priced at 50 cents) in four designs, in a pressure-sensitive adhesive (PSA) pane of 20 stamps (Item 477600). The stamps will go on sale nationwide April 6, 2018. The STEM Education pane of 20 stamps may not be split or sold individually.
What: 
Followers of the U.S. Postal Service’s Facebook page can view the ceremony live at facebook.com/USPS.
directed to Hall E on the upper level of the Convention Center for the dedication ceremony.
Fred Rogers (1928-2003) was known as a beloved television neighbor to generations of children. His groundbreaking public television series “Mister Rogers’ Neighborhood” inspired and educated young viewers with warmth, sensitivity and honesty.
There are both Digital Color and pictorial first-day postmarks for this issue. In addition, local post offices may use this postmark:
The Digital Color Postmark measures 2.43″ x 1.43″
The pictorial postmark for this issue measures 2.33″ x 1.49″ 
On March 5, 2018, in Springfield, IL, the U.S. Postal Service will issue the Illinois Statehood First-Class Mail stamp (Forever priced at 50 cents) in one design, in a pressure-sensitive adhesive (PSA) pane of 20 stamps (Item 477100). The stamp will go on sale nationwide March 5, 2018.
The Digital Color Postmark measures 1.86” x 1.45”
The pictorial postmark for this issue measures 2.73” x 1.49” 
The special (local) postmark measures 2.64” x .94”
On each row of stamps, the third and fourth stamps repeat the first two designs. The selvage also features the transparent deep-sea comb jelly (photo by Gregory G. Dimijian) and the firefly squid (photo by Danté Fenolio). Art director Derry Noyes designed the stamps and selvage from existing photographs.
e postmark by mail. They may purchase new stamps at their local Post Office, at The Postal Store website at 
There are 7 
The stamps are unveiled at the Sunrise Theater in Fort Pierce, FL. Left to right, Jeffrey C. Williamson, Chief Human Resources Officer and Executive VP, USPS; Jill Roberts, News Director, WQCS Public Radio; Dr. Edith Widder, CEO and Senior Scientist, Ocean Research & Conservation Association (ORCA); Taylor F. Lockwood, stamp photographer. 
Williamson and other dignitaries sign autographs after the ceremony. 
A set of 10 first day covers canceled at the event. 
The Digital Color Postmark measures 2.97” x 1.38”
The pictorial postmark for this issue measures 2.91” x 1.47”
On February 9, 2018, in Ft. Lauderdale, FL, the U.S. Postal Service® will issue the U.S. Flag 2018 First-Class Mail® stamp (Forever® priced at 50 cents) in one design, in a pressure-sensitive adhesive (PSA) booklet of 20 stamps (Item 678200) produced by security printers Ashton Potter (USA) Ltd (APU) and Banknote Corporation of America (BCA), and a PSA Coil of 100 stamps (Item 749700) produced by security printers APU and BCA. The stamp will go on sale nationwide February 9, 2018.
It measures 2.95” x .95”.
Background: 
It measures 2.41” x 1.47”. Here is the design for the pictorial first-day postmark: 
It measures 2.95” x 1.40″. Here is the design for the “special” postmark local post offices may use for this issue: 
It measures 2.82” x 1.48”
Updated February 28th:
The Year of the Dog stamp is the 11th of 12 stamps in the Celebrating Lunar New Year series. The Year of the Dog begins Feb. 16, 2018, and ends Feb. 4, 2019.
What:
It measures 3” x 1.64”.
It measures 2.86”x 1.30”. This is the first Lunar New Year stamp issued by the U.S. with a pictorial first-day “rubber composition” postmark.
It measures 2.9” x 1”
Background:
It measures 3″ x 1.56″. Here is the design for the B&W pictorial first-day postmark:
It measures 2.85″ x 1.63″
