House Speaker Nancy Pelosi (D-CA) has encouraged all members of the House to be present in person for a special session on Wednesday night, but hasn't told them why or what will be done during that session. 

"We are ending a very special session of the 117th Congress with legislation that makes progress for the American people as well as support for our Democracy," the letter reads. "Please be present for a very special focus on Democracy Wednesday night."

The Washington Examiner pointed out that the session coincides with a planned trip by Ukrainian President Volodomyr Zelensky to the U.S. that includes a visit to the Capitol.

Zelensky's trip to Washington was still up in the air at the time of Pelosi's announcement on Tuesday, but has since been confirmed.

Zelensky visit confirmed

The Wall Street Journal reported that Zelensky will meet with President Joe Biden during the day on Wednesday and will address a joint session of Congress on Wednesday night.

Zelensky will discuss his efforts to fight back against Russia since its invasion of Ukraine in February.

Some in the U.S. and Ukraine fear that Russia will intensify its attacks against Ukraine in the new year, and Ukraine is also dealing with massive disruptions to its power grid because of the attacks.

The Biden administration said the visit is intended to give “an important injection of momentum” to Ukraine's war effort and to assure continued U.S. support of the effort.

Visit was Biden's idea

Zelensky's visit was apparently Biden's idea, conceived after a phone call the two had on December 11.

Biden issued a formal invitation a few days later with the offer for an extended meeting with Biden and his national security team, as well as the opportunity to address the joint Congress.

“The visit will underscore the United States’ steadfast commitment to supporting Ukraine for as long as it takes, including through the provision of economic, humanitarian, and military assistance,” White House press secretary Karine Jean-Pierre said about the visit.

“On my way to the US to strengthen resilience and defense capabilities” of Ukraine, Zelensky wrote on Twitter early Wednesday, adding, “I will also have a speech at the Congress and a number of bilateral meetings.”

U.S. giving Ukraine Patriot missiles

For his part, Biden is expected to announce that the U.S. will give Ukraine a battery of its Patriot missiles to help it combat the attacks by Russia on its power grid, which have used missiles acquired from Iran.

The Ukrainian military will be trained in Germany to operate the missiles, which may be used against Russia by spring.