The Capitol dome at night. Click on picture for larger image

The dome of the Capitol is lit up every night of the year, as are the House and Senate steps, and the grounds. The last time the Capitol was left "in the dark" was during World War II, for black-out purposes. I think you may be referring to the "Tholos", the large lantern located in the space below the Statue of Freedom, and above the Rotunda. The Tholos is surrounded by 12 Corinthian columns and remains illuminated when either the House or Senate, or both, are in session. When neither is meeting, the lantern is not lit.