Cuban Missile Crisis: October 20, 1962

 From the Nuclear Files timeline:

9:00A.M.: ExComm [the Executive Committee of the National Security Council, an ad hoc group formed to make recommendations to the president] meetings continue at the State Department. Final planning for the implementation of a naval blockade is completed, and Theodore Sorensen's draft speech for President Kennedy is amended and approved. As McNamara leaves the conference room, he reportedly phones the Pentagon and orders four tactical squadrons to be readied for a possible airstrike on Cuba. McNamara explains to an official who overhears the conversation, " If the president doesn't accept our recommendation, there won't be time to do it later." 

2:30P.M.: President Kennedy meets with the full group of planning principals. He notes that the airstrike plan as presented is not a "surgical" strike but a massive military commitment that could involve heavy casualties on all sides. As if to underscore the scale of the proposed U.S. military attack on Cuba, one member of the JCS reportedly suggests the use of nuclear weapons, saying that the Soviet Union would use its nuclear weapons in an attack. President Kennedy directs that attention be focused on implementing the blockade option, calling it the only course of action compatible with American principles. The scenario for the full quarantine operation, covering diplomatic initiatives, public statements, and military actions, is reviewed and approved. Kennedy's address to the nation is set for October 22, at 7:00P.M.
...
Late night: James Reston, Washington Bureau Chief for the New York Times, phones George Ball and McGeorge Bundy to ask why there is such a flurry of activity in Washington. Reston is given a partial briefing on the Cuban situation but is requested to hold the story in the interests of national security.

From the National Geospatial Intelligence Agency:

Mariel, on the northwestern coast of Cuba, served as a major port of entry for Soviet goods and as a Cuban naval base. NPIC analysts monitored imagery of the port to gauge whether additional Soviet materials were brought in to add to the already growing threat. The Soviets attempted to send four “Foxtrot” diesel submarines to Cuba during the crisis. Intelligence gathering provided by NPIC and other agencies resulted in the U.S. Navy successfully forcing three subs to the surface before turning them around, with the fourth turning back in mid-journey, still submerged.