It is not the moon, but the ZODIAK signs you go by.
Each part of the body is assigned a Zodiak sign.
They say that when the "blood" is towards the head, it's bad to wean or castrate. You want to do that when the blood is towards the feet.
Here goes!
Head=Aries
Neck=Taurus
Arms=Gemini
Breast=Cancer
Heart=Leo
Belly=Virgo
Reins=Libra
Secrets=Scorpio
Thighs=Sagittarius
Knees=Capricorn
Legs=Aquarius
Feet=Pisces
If you wean gradually. start way ahead of the "date" given to wean. Then wean totally during the "wean" time.
We used to wean gradually and it was hard on the foals and mares. Once in a while it went smoothly, but those were the times I accidentally was weaning by the sign! Now we wean cold turkey by the signs and it goes very well for mares and foals. The mare's bags go down in two days (here) and foals don't run and cry. We do what works for us.
I like to start the wean if it's at least a day or two into "thighs".
Here is Aug.
Aug. 14 SAG
Aug. 15 SAG
Aug. 16 CAP
Aug. 17 CAP
Aug. 18 AQU
Aug. 19 AQU
Aug. 20 PSC
Aug. 21 PSC
Since Pisces is the "Feet" that is the best time. If you tried to start weaning on the 22nd, it would be back at the head and it will go roughly.
They say gelding during the right time that they bleed less too.
I have 3 foals I'm going to start to wean Aug. 15th or 16th.
I hope I made sense, I have not been to bed yet last night and I can't see straight.