WASHINGTON, Nov. 29 — Marking the 14th anniversary of legislation that allowed gay men and lesbians to serve in the military but only if they kept their orientation secret, 28 retired generals and admirals plan to release a letter on Friday urging Congress to repeal the law.

“We respectfully urge Congress to repeal the ‘don’t ask, don’t tell’ policy,” the letter says. “Those of us signing this letter have dedicated our lives to defending the rights of our citizens to believe whatever they wish.”

The retired officers offer data showing that 65,000 gay men and lesbians now serve in the American armed forces and that there are more than one million gay veterans.

“They have served our nation honorably,” the letter states.

The letter’s release comes as rallies are scheduled on the Mall by groups calling for a change in the law, which is known as “don’t ask, don’t tell” because it bars the military from investigating soldiers’ sexual orientation if they keep it to themselves.