Yes, it has to do with the zodiac signs and specifically the gravitational pull of the moon. There is truth to the saying emergency rooms are more crowded on a full moon.
You always want to castrate on dates below the groin, going downward (see chart). Each month has a specific window of ideal days for this. Man of The Signs Zodiac Moon's Sign Calendar Astrological Place Chart
We always try to go by the signs when doing any type of elective weaning, and castration. The dates vary depending if you are weaning, castrating, etc. Calves weaned according to the dates only bawl about 2 days, and there is less sickness. If weaned at the wrong time, they bawl 3-4 days, and they may be a little more puny/more likely to get sick. Same with the cows, they bawl longer for their babies.
Also, I have found that when gelding a colt in this time frame, it's an excellent opportunity to back the colt for the first time. I was riding my colt in 2 days. He had no swelling, no discomfort, but wasn't feeling quite good enough to buck me off. Of course I had done the proper ground work before hand, but was waiting for him to be slightly vulnerable. He healed beautifully, and I never looked back with his training. Never skipped a beat.
Cooler weather def. helps with swelling and flies, but not doing it during the right signs can be as bad or worse than castration during hot, humid fly season.
Vets downplay this, as they can't schedule things around the signs, but the vets we know, know the truth of the signs.
This link explains it well, and we operate the same way: http://www.stallingspainthorses.com/...moon-signs.htm
Just check the farmers almanac, and make sure moon signs are in the "thighs" going "down".