There are different schools of thought on this. Here is one that suggests best to flush on a full/waning moon:
"The main part of the cleanse is best done over a weekend, when you have enough time to rest, and preferably during the days of full moon or waning moon."
I can see the argument for both, especially the waning moon, and do not think about it much. Pay attention to your flush results during various moon cycles and you will find out what is best for you, instead of relying on what others tend to believe...
Maybe keep a log?