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There might be no reason but I doubt the date had simply no reason.

I believe there may have been personal significance to Neil Armstrong. Alternatively picking a summer date may have been safer.

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    $\begingroup$ What significance to Armstrong are you thinking of? I'm fairly sure the personal feelings of the crew haven't ever set any launch dates. $\endgroup$ – Russell Borogove Aug 16 '16 at 13:07
  • $\begingroup$ I think they were shooting for a 4th of July landing and technical delays made them miss that target. $\endgroup$ – Howard Miller Aug 16 '16 at 17:26
  • $\begingroup$ @RussellBorogove I can't find the source but his first flight as a pilot occurred as kid on July 20th. $\endgroup$ – William Aug 16 '16 at 18:45
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    $\begingroup$ @howardmilller As Hobbes' answer explains, sun position forced landing at a certain point of the month; a July 4th Earth return date might have been possible in a 1968 landing mission, but not a July 4th landing in '69. Are you thinking of Viking 1? $\endgroup$ – Russell Borogove Aug 16 '16 at 19:57
  • $\begingroup$ There were a lot of practical considerations that went into something as mundane as the crew egress order for EVA, or the mission numbering. (The statement in the movie Apollo 13 was very much indicative of the attitude at the time: "Why 13?" "It comes after 12.") As suggested by Russell, why would the landing date be any different at all? $\endgroup$ – a CVn Aug 17 '16 at 10:49
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The reasons for that date were:

There were a number of considerations which determined the launch windows for a lunar landing mission. These considerations included illumination conditions at launch, launch pad azimuth, translunar injection geometry, sun elevation angle at the lunar landing site, illumination conditions at Earth splashdown, and the number and location of the lunar landing sites.

The time of a lunar landing was determined by the location of the lunar landing site and by the acceptable range of sun elevation angles. The range of these angles was from 5° to 14° and in a direction from east to west. Under these conditions, visible shadows of craters would aid the crew in recognizing topographical features. When the sun angle approached the descent angle, the mean value of which was 16°, visual resolution would be degraded by a “washout” phenomenon where backward reflectance was high enough to eliminate contrast. Sun angles above the flight path were not as desirable because shadows would not be readily visible unless the sun was significantly outside the descent plane. In addition, higher sun angles (greater than 18°) could be eliminated from consideration by planning the landing one day earlier where the lighting is at least 5°. Because lunar sunlight incidence changed about 0.5° per hour, the sun elevation angle restriction established a 16-hour period, which occurred every 29.5 days, when landing at a given site could be attempted. The number of Earth-launch opportunities for a given lunar month was equal to the number of candidate landing sites.

The time of launch was primarily determined by the allowable variation in launch pad azimuth and by the location of the Moon at spacecraft arrival. The spacecraft had to be launched into an orbital plane that contained the position of the Moon and its antipode at spacecraft arrival. A 34° launch pad azimuth variation afforded a launch period of 4 hours 30 minutes. This period was called the “daily launch window,” the time when the direction of launch was within the required range to intercept the Moon.

Two launch windows occurred each day. One was available for a translunar injection out of Earth orbit in the vicinity of the Pacific Ocean, and the other was in the vicinity of the Atlantic Ocean. The injection opportunity over the Pacific Ocean was preferred because it usually permitted a daytime launch.

So they basically had one launch opportunity per month. I suspect they chose the earliest possible opportunity that could be achieved safely.

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  • $\begingroup$ If Apollo 11 had failed to achieve a landing and safe return for the astronauts, nearly half a year also gave NASA and their contractors a chance to fix the problem and still make the deadline of New Year's Eve 1969. Though perhaps not official, I suspect that thought was on quite a few peoples' minds. $\endgroup$ – a CVn Aug 15 '16 at 19:40
  • $\begingroup$ Is this which depending on the time zone is july 20th just a coincidence jpl.nasa.gov/spaceimages/details.php?id=pia17172 $\endgroup$ – William Nov 3 '16 at 22:31

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