RAWALPINDI, Pakistan, June 17— A Government spokesman announced today that Yahya Khan would disclose on June 28 his plan for returning the nation to political normalcy.

Pakistan held her first free national election in 23 years of independence last Dec. 7. At that time a National Assembly was elected.

But on March 1, President President Agha Mohammad Yahya Khan Indefinitely postponed the convening of the National Assembly as a result of pressure from Zulfikar Ali Bhutto, whose leftist Pakistan People's party had won a majority of seats in West Pakistan.

The overall majority had been won by East Pakistan's Awami League, which favored autonomy for the eastern wing. Some of its leaders advocated outright secession.

On March 25 the army moved into East Pakistan to crush the separatist movement. The nation remains under martial law.