Tunes published in 1926 will be entering public domain in 2022

As of January 1, 2022, U.S. copyright will expire for works published in 1926, including the following songs: 

(What Can I Say) After I Say I'm Sorry (Walter Donaldson and Abe Lyman)

Baby Face (Harry Akst)

Birth of the Blues ((Ray Henderson)

Black Bottom (Ray Henderson) - See note below

Black Bottom Stomp (Jelly Roll Morton) - See note below

Blue Room (Rodgers and Hart)

Blue Skies (Irving Berlin) - Sheet music pub. 1927, but first performed 1926

Bye Bye Blackbird (Ray Henderson)

East St. Louis Toodle-Oo (Duke Ellington and Bubber Miley)

If I Could Be With You (One Hour Tonight) (James P. Johnson)

Mountain Greenery (Rodgers and Hart)

Muskrat Ramble (Kid Ory) - See note below

Someone to Watch Over Me (George and Ira Gershwin)

When the Red, Red Robin (Comes Bob, Bob, Bobbin' Along) (Harry Woods) 

This year, the big ones would seem to be Birth of the Blues, Blue Room, Blue Skies, Bye Bye Blackbird, and Someone to Watch Over Me. 

In classical music, notable pieces entering public domain are Bartok's Piano Concerto #1, Copland's Piano Concerto, and Puccini's Turandot.

For more popular, jazz, and classical pieces entering the public domain, see the Wikipedia page 1926 in Music.

United States copyright law is quite restrictive as compared to many other countries. According to the provisions of the Copyright Act of 1976 and the Copyright Term Extension Act of 1998 (aka "Mickey Mouse Protection Act"), works published or registered before 1978 remain under copyright for 95 years.

With the passage of the 1998 law, the cutoff date for works entering the public domain became 1922, with any works published in 1923 or later remaining under copyright. Beginning in 2019, however, the clock began running again, with each new year bringing one more year of songs and other works into the public domain. Over the next 20 years or so, most "Golden Age" jazz standards will lose copyright protection.

Many other countries have shorter terms of copyright; one common formula is the life of the author plus fifty years (see this table). For example, in Canada you can record pieces written by Wes Montgomery (d. 1968) John Coltrane (d. 1967), Igor Stravinsky (d. 1971), or Louis Armstrong (d. 1971).

Looking ahead, Mickey Mouse will become fair game in the US in 2024, unless Congress is somehow persuaded to change the present copyright law. 

Notes:

"Black Bottom" was actually a dance, not one particular song. There were several popular songs that included "Black Bottom" in the title. The first was "Original Black Bottom Dance," copyrighted in 1919 by Perry Bradford (it included instructions for the steps in the dance). Yet another song, recorded by Ma Rainey in 1927, "Ma Rainey's Black Bottom," provided the title for a recent stage play and film. The term "Black Bottom" referred to a neighborhood in Detroit. A short, informative history of the dance can be found here.

"Muskrat Ramble" has an interesting copyright history, detailed in its Wikipedia entry:

• Kid Ory received no royalties until 1947, when Barney Bigard took him to the offices of the company that held the copyright. At that point, the company paid him retroactive royalties, and quarterly royalties thereafter.

• In 1950, Ray Gilbert wrote lyrics to "Muskrat Ramble" without Ory's permission, and in 1951 demanded that ASCAP issue a decision entitling him to a share of royalties. In 1956 ASCAP decided in Gilbert's favor, and decreed that he was entitled to a one-third share, retroactive to 1950 - in my opinion, setting a very questionable precedent.

• In 2001 Babette Ory, Kid Ory's daughter, sued Country Joe McDonald for his use of a section of Muskrat Ramble in Country Joe's Feel Like I'm Fixin' to Die Rag (at 0:20 in the Ory link, 0:39 in the Country Joe link). The court ruled against Babette and in favor of McDonald. From Wikipedia:

This suit was dismissed due to the lateness of the filing. Since decades had already passed from the time McDonald composed his song in 1965, Ory based her suit on a new version of it recorded by McDonald in 1999. Judge Nora Manella, then of the United States District Court for the Central District of California, upheld McDonald's laches defense, noting that Ory and her father were aware of the original version of the song, with the same questionable section, for some three decades without bringing a suit. This ruling was upheld by the United States Court of Appeals for the Ninth Circuit in 2005, and Ory was also ordered to pay McDonald's substantial attorneys' fees.

 

Copyright Law of the United States (Wikipedia)