President Reagan has promised Yelena Bonner, wife of dissident Soviet scientist Andrei Sakharov, that the United States "will continue to do everything possible" to win her husband's freedom and "advance the principles for which he has sacrificed so much."

Reagan made the pledge in a letter to Bonner made public yesterday at a ceremony on Capitol Hill to mark Sakharov's 65th birthday.

The president also issued a proclamation declaring yesterday "Andrei Sakharov Day" and citing his confinement in the closed city of Gorki as evidence of "the overall grim human rights situation that continues to prevail in the Soviet Union."

The letter and proclamation were seen as an attempt by the administration to reiterate U.S. concern for Sakharov despite Reagan's refusal to meet Bonner during a recent White House visit, to avoid provoking Moscow.

Bonner, who has been in the United States to visit her children and receive medical treatment, is to return to the Soviet Union in a few days.

Through a translator, she said she viewed Reagan's letter "as an expression of concern for my husband, and I hope his concern will not be in vain."

About 200 persons attended the birthday celebration, sponsored by the House Foreign Affairs Committee.

Asserting that "we cannot remain silent" about Sakharov's fate, committee Chairman Dante B. Fascell (D-Fla.) said, "What this ceremony today says to Andrei Sakharov . . . and to the leaders of the Soviet Union . . . is that Americans have not forgotten him and will not forget him."