Answer: December 22, 1963

I'm going to be absurdly specific, and then I'm going to give a longer answer that explains why the answer above is wrong.

References for this answer come from the CIA's FOIA database. Notably, refer to "The 1965 Sino-Soviet-Vietnamese Controversy Over Soviet Military Aid to North Vietnam" (20 Dec. 1965); "Current Intelligence Staff Study North Vietnam and Sino-Soviet Relations" (4 Mar. 1962); "The Sino-Soviet Dispute on Aid to North Vietnam (1965-1968)" (30 Sep. 1968); and "Anomalies in Soviet Shipping to North Vietnam During the First Half of 1969" (July 1969).

In January 1964, American recon aircraft were making regular runs over North Vietnam in support of the American military missions in Laos and South Vietnam. While this is months before the Gulf of Tonkin incident, the United States was already strongly committed to the region. So were the Soviet Union and China, but all three countries were focused as much on Laos as they were on Vietnam.

On a flight early in the month, this recon aircraft makes the first sighting of Soviet self-propelled antiaircraft guns in North Vietnam. The weapons are presumed to have been delivered by a Soviet freighter that docked in Haiphong on Dec. 22, 1963.

As early as 1963, according to a Chinese source, the USSR had promised North Vietnam "rocket units" (presumably surface-to-air missiles) and "one air group" of MiG-17s. Khrushchev evidently reneged on this promise, as these items did not arrive in the country until 1965. Even though the Soviet Union did not begin shipping heavy equipment to North Vietnam until this point, the two countries already had extensive interactions, and the Soviet Union had been shipping lighter equipment to North Vietnam for years..

The Soviet Union opened its first embassy in Hanoi in 1954. Before this time, North Vietnam had been much closer to the People's Republic of China than it had been to the Soviet Union. Beginning in the late 1950s, this started to change for a variety of reasons. By 1959, it was clearly starting to favor improved relations with the USSR.

In January 1960, Le Duan announced that North Vietnam would switch from agricultural emphasis to heavy industrial development in its first Five-Year Plan (1960-1965) and it would learn "especially from the USSR" in this approach. A Soviet scientific and technical mission arrived in spring 1960 to that end.

Following this mission, the Soviet Union granted North Vietnam substantial financial credits to buy agricultural and industrial equipment. China did as well, but owing to its internal difficulties at this time, the Soviet offer was more attractive to the North Vietnamese.

By the end of the year, Soviet advisers and supplies were flooding into Laos in the largest Soviet airlift since WWII. Between 1960 and 1962, the Soviet Union operated an airlift from Irkutsk to Hanoi and on to Vientiane with the cooperation of the Chinese government, which permitted the overflight of its territory.

With North Vietnam intensively involved in that conflict, some of the supplies were siphoned off by the Vietnamese government. As late as summer 1962, however, the CIA had not found any Soviet bloc weaponry in use by fighters in South Vietnam.

Soviet shipments of military aid remained small until about the time of the Gulf of Tonkin Incident. At that time, the North Vietnamese began urgently requesting anti-air defenses. These were slow in arriving for three main reasons: The Chinese did not allow another significant airlift, the Soviets were concerned about the vulnerability of shipping to a Cuban-style blockade, and the Chinese and Soviets had a pretty severe disagreement in 1965 over rail shipments.

Because of these three factors, the first surface-to-air missile site (which began construction in March 1965) was not complete until July 24 of that year, when its Soviet crew fired its first missile. Three days later, the U.S. bombed the site for the first time, and the dog-and-cat fight between American aircraft and Vietnamese SAMs was underway.