The U.S. Africa Command spokesman said the airstrike had no link to the anniversary.
The United States, which also has targeted a small number of fighters linked to the Islamic State group in northern Somalia, has increased its military presence in the long-chaotic Horn of Africa nation since early 2017 to about 500 personnel after President Donald Trump approved expanded military operations.
Al-Shabab, which seeks to establish an Islamic state in Somalia, continues to hold parts of the country's south and central regions after being chased out of Mogadishu several years ago. The group, estimated at several thousands of fighters, still carries out deadly attacks against high-profile targets such as hotels and checkpoints in the capital and other cities. It also remains a threat in parts of neighboring Kenya.
While some observers have questioned the effectiveness of airstrikes by the U.S. and Kenya and raised concerns about civilians being killed, the U.S. military maintains that they "reduce al-Shabab's ability to plot future attacks, disrupt its leadership networks and degrade its freedom of maneuver within the region."
Somali forces in the next few years are expected to take over responsibility for the country's security from an African Union peacekeeping force, which already has started withdrawing hundreds of personnel.
The U.S. says it continues to support that transfer even as some U.S. military officials have joined AU officials and others in expressing concern that Somalia's forces are not yet ready.