When a national celebration conflicts with an episode of official mourning, what happens? There's no general rule, so far as we could find in a quick and inconclusive bit of reasearch yesterday, although the inaugural celebration seemed to be primary.

Rep. Gillis Long (D-La.) had died overnight. By law, when a member of Congress dies, flags in the capital and in his home state or district fly at half staff. At almost any other time, flags would have been so flown. Through most of the day, all flags on government buildings -- including those over the Capitol shown on television -- stayed at full staff. The inaugural celebration apparently took precedence, which this column doesn't contest.

Rep. Long was a member of the partisan opposition, so it was especially gracious for President Reagan, at the outset of his inaugural speech, to invoke a period of silent prayer for the congressman.

Later in the day, after the inauguration had ended, government flags were lowered to half staff, including the one over the White House's north portico. It could not be learned yesterday who made the Solomon-like flag decision. The presidential press office said it wasn't made at the White House.