All works published in the United States prior to 1923 are considered public domain. Why 1923? Because of the Sonny Bono Copyright Term Extension Act and Disney's mascot Mickey Mouse.
The Copyright Act of 1909 initially stipulated that all works were granted protection for 28 years, with the option to renew for an additional 28 years. After that 56-year period, the work would enter the public domain, according to the United State Copyright Office.
"Steamboat Willie," the very first Mickey Mouse cartoon was published in 1928 and thus was slated to become public domain in 1984. However, Disney lobbied Congress in 1976 to extend copyright law to a term of 75 years. With that extension, the company's copyright would expire in 2003.
This wasn't the only time that Disney advocated for a copyright extension. In the late '90s, the company once again lobbied congress and in 1998 the Sonny Bono Act was signed into effect by President Bill Clinton. This act extended all existing copyrighted material by an additional 20 years.
Works that had been published in 1922 were unaffected because their 75-year copyrights had already expired by the time the extension was enacted. Works published in 1923, which had not expired in 1998, were granted the 20-year extension.
If Congress does not elect to extend the length of copyright, these works will begin to enter public domain in 2018. However, it would not be unexpected for Disney to seek another copyright extension for their famous rodent. In fact, brand experts in 2008 valued Disney's mouse at more than $3 billion, according to the Los Angeles Times. No doubt that number has only increased in the last seven years. (Disney didn't respond to a request for comment.)