Since I was a teen, I remember reading in The Farmer's Almanac about "Best Days". They are influenced by the phases of the moon. Farmers have been following the "Best Days" guide for planting, pruning, harvesting etc, but they also have best days for many other activities, including cutting hair to encourage growth or to discourage growth. It's easier than ever now to look up the best days since The Almanac has a great website.
Morrocco Method has taken it a step further and has best days to trim for thickness, length, to touch up your roots or to all-over beautify.
I find all of this fascinating, and people all over the world plan many tasks based on moon phases.
While there is really no science to this, I do tend to look up the best days for trimming when I do my regular trims. Does it work? I don't know. But it can't hurt, especially since I trim anyway. Who am I to argue with centuries of folk wisdom? Call me crazy, but I don't care.
Here is the link to Almanac.com's best days. Here is the link to Morrocco Method's Lunar Chart and Hair Care. Even if you don't believe in the idea, it is fun to think about.